Electrical machines for aircraft power and propulsion systems

ABSTRACT

An electrical propulsion unit (EPU) for a vertical take-off and landing (VTOL) aircraft is provided. The EPU includes a propeller or a fan and an electric motor. The electric motor includes a stator comprising coils configured to carry current and a rotor arranged to interact with the stator. The rotor is configured to generate a torque for driving rotation of the propeller or the fan. The coils of the stator have a cumulated conductor volume V conductor  The stator, the rotor, or the stator and the rotor include a flux guiding iron material configured to guide magnetic flux generated by the coils of the stator, wherein the flux guiding iron material has a cumulated iron volume V iron , and wherein the electric motor has a machine parameter Γ that is greater than or equal to 0.25, Γ being defined as: Γ=V conductor /V iron .

The present patent document claims the benefit of United Kingdom PatentApplication No. 2300383.3, filed Jan. 11, 2023, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to rotary electrical machines for use inaircraft electrical power systems, and particularly, but notexclusively, to rotary electrical machines for use in the propulsionsystems of vertical take-off and landing (VTOL) aircraft. The disclosurealso relates to aircraft and aircraft electrical propulsion units (EPUs)including such rotary electrical machines.

BACKGROUND

In aerospace, the desire to reduce greenhouse emissions combined withimprovements in the underlying electrical technologies has createdinterest in new types of aircraft and aircraft propulsion systems. Thisincludes purely electric aircraft having an onboard power source thatincludes batteries and/or fuel cells. The purely electric aircraftincludes one or more electrical propulsion units (EPUs) in which anelectric motor drives rotation of a propulsive propeller or fan. Thisalso includes hybrid-electric aircraft having an onboard power sourcethat include one or more engines (e.g., gas turbine engines). An engineof a hybrid-electric aircraft may drive an electric generator to provideelectrical power for an EPU, or the engine may provide propulsive thrustwith driving assistance from a motor coupled to a shaft of the engine.

Electric and hybrid-electric aircraft may be of a conventional type(e.g., Conventional Take-Off and Landing [CTOL]) or may have VerticalTake-Off and Landing (VTOL) capabilities. VTOL aircraft may be used forUrban Air Mobility (UAM) applications. UAM refers to the use of VTOLaircraft to transport a relatively small number of passengers relativelyshort distances (e.g., of the order of tens of, or perhaps a fewhundred, kilometers, such as in or between urban environments).

The design requirements of electrical machines (e.g., motors andgenerators) used in aerospace applications are somewhat different fromthose in other industries, due in part to the criticality of theirfunction and the resulting requirements for certification. For example,the expected failure rate is to be low, and the tolerance of themachine, and indeed the wider tolerance of the system, to a failure isto be high. While many of these design requirements remain in place forthe electrical machines of the new aircraft types mentioned above, theelectrical machines used for the new aircraft types have new designrequirements that cannot be met by established aerospace electricalmachine designs. For example, the widely used radial flux wound fieldmachines and radial flux permanent magnet alternators (PMAs) used togenerate electrical power from, and in some cases start, aircraft gasturbine engines cannot meet the power density or torque densityrequirements of an EPU for a VTOL aircraft. It will therefore benecessary to adopt new aerospace electrical machine designs for thesenew aircraft.

Until superconducting electrical machine technology matures to a pointwhere such technology may be used in safety critical aerospaceapplications, it is likely that permanent magnet electrical machineswill be used for electric and hybrid-electric aircraft due to theirfavorable power density compared with other machine types. Variouspermanent magnet electrical machine designs have been proposed foraerospace applications, most of which are of the radial flux type.However, work remains to further improve and refine these designs foraerospace applications. This includes, for example, reducing theirfailure rate and improving their fault tolerance while also optimizingtheir levels of torque production, mass, and efficiency.

SUMMARY

The scope of the present disclosure is defined solely by the appendedclaims and is not affected to any degree by the statements within thissummary. The present embodiments may obviate one or more of thedrawbacks or limitations in the related art.

Herein, unless specified otherwise, where a value of a measured ormeasurable quantity is dependent on measurement conditions such asambient temperature and pressure, the measurement conditions are ISA(International Standard Atmosphere) sea level conditions. ISA sea levelconditions correspond to an ambient temperature of 288 K (15° C., 59°F.) and an ambient pressure of 101.325 kPa (1,013.25 mbar, 14.7 psi).

According to a first aspect, an electrical machine for an aircraftelectrical power system is provided. The electrical machine includes astator having coils for carrying current and a rotor arranged tointeract with the stator to produce a torque for driving the rotor torotate or to generate electrical power in the coils of the stator.

According to a second aspect, an aircraft electrical power systemincluding an electrical machine according to the first aspect isprovided. The rotor of the electrical machine is mechanically coupled,directly or indirectly, to a rotary shaft of a propulsor of theaircraft. The rotary shaft may be an engine shaft (e.g., a shaft orspool of a gas turbine engine) or a shaft of an electrical propulsionunit (EPU). The electrical power system may be a purely electricaircraft power system or a hybrid-electric aircraft power system. Theelectrical power system may be a power and propulsion system.

According to a third aspect, an electrical propulsion unit (EPU) for anaircraft is provided. The EPU includes a propeller or fan and anelectrical machine according to the first aspect. The electrical machineis configured as an electric motor, and the rotor of the electricalmachine is mechanically coupled, directly or indirectly, to thepropeller or fan and arranged to drive rotation of the propeller or fan.

According to a fourth aspect, an aircraft including the electricalmachine of the first aspect, the electrical power system of the secondaspect, or the EPU of the third aspect is provided. In one group ofembodiments, the aircraft is a Vertical Take-Off and Landing (VTOL)aircraft including one or more of the EPUs. The VTOL aircraft may be apurely electric aircraft (e.g., electric VTOL or eVTOL aircraft) or ahybrid-electric VTOL aircraft. In other embodiments, the aircraft may bea Conventional Take-Off and Landing (CTOL) aircraft. The aircraft mayinclude a plurality of the EPUs.

In one group of embodiments, the electrical machine is a transverse fluxelectrical machine.

The stator and the rotor of the transverse flux electrical machinedefine magnetic circuits along which magnetic flux flows. The magneticflux paths may be three-dimensional.

The stator of the transverse flux electrical machine may include fluxguiding stator iron defining one or more stator slots housing the statorcoils. In some examples, there are a plurality of stator slots, and eachof the stator slots houses one of the coils. Each of the one or morestator slots may be circumferentially extending, and current may flowthrough the coil in a circumferential direction relative to an axis ofrotation of the electrical machine. The three-dimensional magnetic fluxpaths may flow around (e.g., helically around) the stator slots. The oneor more stator slots may be annular in shape.

Each of the one or more stator slots may be an open slot. The term “openslot” refers to a slot that is not hermetically sealed or fullyenclosed, such that the coil is exposed to the environment of thestator. For example, the flux guiding stator iron may include orificesor spaces that expose the coil to the environment of the stator.

Each of the one or more stator slots may have an angular extent in thecircumferential direction of at least 10 degrees, at least 20 degrees,at least 30 degrees, or at least 40 degrees. In an example, each of theone or more stator slots has an angular extent in the circumferentialdirection in a range of 25 to 65 degrees.

Each of the one or more stator slots and each coil may include a firstportion extending in a circumferential direction and a second portionspaced apart from the first portion and extending in a circumferentialdirection. In one group of examples, the first portion and the secondportion are radially spaced apart. In another group of examples, thefirst portion and the second portion are axially spaced apart. Each ofthe one or more stator slots and each stator coil may be banana shaped,(e.g., may approximate the shape of a circumferential segment of anannulus).

For each of the one or more stator slots, the flux guiding stator ironmay include circumferentially arranged flux guiding stator elements.Each of the one or more stator slots may be associated with two sets offlux guiding stator elements: a first set of flux guiding statorelements defining the first slot portion; and a second set of fluxguiding stator elements defining the second slot portion.

A stator slot of the one or more stator slots, the associated statoriron (e.g., two sets of stator elements), and the coil housed within theslot may be referred to as a stator segment or stator module. The statorsegment or stator module may be a replaceable module that may be easilyexchanged in case of maintenance or failure, for example. A plurality(e.g., six) of stator segments may be arranged along the circumferenceof the stator.

Each stator element may be an elongated and at least partially curvedshaped. Each stator element may be C-shaped or claw-shaped. Each fluxguiding stator element may have a body portion and a pair of projectionsthat project from the body portion (e.g., a pair of pole heads). In oneexample, the stator elements are oriented so the projections projectradially from the body portion. In another example, the stator elementsare oriented so the projections project axially from the body portion.The projections of circumferentially adjacent flux guiding statorelements may be arranged in radial or axial opposition so that thecircumferentially adjacent flux guiding stator elements define a slotcross-section perpendicular to the circumferential direction.

Each stator slot or stator slot portion may have a circular, polygonal,rectangular, or other cross section in a plane perpendicular to adirection of current flow.

Each stator slot may have a slot fill factor, defined as a cumulatedcross-sectional area of a current carrying coil in a slot divided by thecross-sectional area of the slot housing the coil. The slot fill factormay be less than or equal to 70%. The slot fill factor may be less thanor equal to 60%, less than or equal to 50%, less than or equal to 40%,or less than or equal to 35%. The slot fill factor may be greater thanor equal to 10%, greater than or equal to 15%, greater than or equal to20%, or greater than or equal to 25%. The slot fill factor may in therange of 10% to 50% or in the range of 20% to 40%.

The stator may include at least two slots and associated coils per phaseof the electrical machine. Coils of the same phase may be electricallyconnected together (e.g., in series or parallel), and coils of the samephase may be circumferentially spaced apart by 2π/N_(c) radians, whereN_(c), is the number of coils per phase. The stator coils of differentphases may be connected in a star configuration or in a deltaconfiguration.

Each stator coil may have a plurality of winding turns formed from acontinuous length of conductor. Each stator coil may include a pluralityof (e.g., two) winding packages, each winding package including a subsetof winding turns of the plurality of winding turns. The winding packagesof the coil may be spaced apart (e.g., axially spaced apart and/orradially spaced apart) to define a cooling channel therebetween, throughwhich the flow of air may pass.

The rotor of the transverse flux electrical machine may include aplurality of permanent magnets (e.g., rotor magnets) circumferentiallydistributed about the rotor. The plurality of permanent magnets maydefine a plurality of circumferentially arranged rotor magnet poles,with circumferentially adjacent poles being of opposite polarity. Theplurality of permanent magnets of the rotor may face and be separatedfrom the stator by an air gap. The air gap may be a radial air gap(e.g., the rotor magnets may be radially spaced from the stator), or theair gap may be axial (e.g., the rotor magnets may be axially spaced fromthe stator).

The rotor may be a dual rotor including a first rotor portion (e.g., aninner rotor portion) and a second rotor portion (e.g., an outer rotorportion), with the stator located between the first rotor portion andthe second rotor portion. In one example, the first rotor portion andthe second rotor portion are a radially inner rotor portion and aradially outer rotor portion, respectively, with the stator locatedradially between the radially inner rotor portion and the radially outerrotor portion. In another example, the first rotor portion and thesecond rotor portion are axially spaced rotor portions with the statorlocated axially between the axially spaced rotor portions. The firstrotor portion may include a first set of permanent magnets, and thesecond rotor portion may include a second set of permanent magnets. Thefirst set of permanent magnets may face and be separated from a firstside of the stator by a first air gap, and the second set of permanentmagnets may face and be separated from a second side of the stator by asecond air gap, the first side and the second side of the stator beingopposing sides. The first air gap and the second air gap (e.g., firstand second magnetic air gaps) may be radial air gaps, or the first andsecond magnetic air gaps may be axial air gaps.

Each set of permanent magnets may include a first group of permanentmagnets and a second group of permanent magnets. The first group ofpermanent magnets may be located opposite a first slot portion of a slotof the stator, and the second group of permanent magnets may be oppositea second slot portion of a slot of the stator.

The rotor may be ironless. Where the rotor is a dual rotor, both thefirst rotor portion and the second rotor portion may be ironless.

The permanent magnets of each set or each group may be arranged in aHalbach array.

The transverse flux electrical machine may be a multi-lane electricalmachine. In other words, the electrical machine may include at least two(e.g., two or four) sub-machines, each sub-machine of the at least twosub-machines having an electrically independent set of stator coils. Forexample, the electrical machine may have a first three-phase sub-machineand a second three-phase sub-machine.

In one example, the multi-lane transverse flux electrical machine has: afirst sub-machine having a first stator and a first rotor arranged tointeract with the first stator; and a second sub-machine having a secondstator and a second rotor arranged to interface with the second stator.Axes of rotation of the first rotor and the second rotor may be coaxial,but the first sub-machines and the second sub-machine are axially spacedapart from each other. The first rotor and the second rotor may bemechanically coupled so that the first rotor and the second rotor rotatetogether.

In another example of a multi-lane electrical machine, a stator iscircumferentially divided into a first sector and a second sector. Thefirst sector includes a first set of stator slots and correspondingstator coils belonging to a first sub-machine. The second sectorincludes a second set of stator slots and corresponding stator coilsbelonging to a second sub-machine. The first sub-machine and the secondsub-machine are arranged to interact with a common rotor.

In yet another example, the multi-lane transverse flux electricalmachine has at least four lanes (e.g., at least four sub-machines). Theelectrical machine has a first stator and a corresponding first rotor,and a second stator and a corresponding second rotor. Axes of rotationof the first rotor and the second rotor are aligned (e.g.,coincident/coaxial), but the first stator and first rotor are axiallyspaced apart from the second stator and the second rotor. The firstrotor and the second rotor may be mechanically coupled so that the firstrotor and the second rotor rotate together. The first stator iscircumferentially divided into a first sector and a second sector. Thefirst sector includes a first set of stator slots and correspondingstator coils belonging to a first sub-machine. The second sectorincludes a second set of stator slots and corresponding stator coilsbelonging to a second sub-machine. The first sub-machine and the secondsub-machine share and interact with the first rotor. The second statoris circumferentially divided into a third sector and a fourth sector.The third sector includes a third set of stator slots and correspondingstator coils belonging to a third sub-machine. The fourth sectorincludes a fourth set of stator slots and corresponding stator coilsbelonging to a fourth sub-machine. The third sub-machine and the fourthsub-machine share and interact with the second rotor.

The transverse flux electrical machine may have a cooling system forremoving heat, including from the stator coils. In one group ofexamples, the cooling system is an air cooling system that uses ambientair to cool the stator coils. In one example, the cooling system is adirect air cooling system in which heat is transferred directly from thestator coils into the cooling air, without an intermediate heatexchanger, to increase a rate of heat transfer from the stator coils tothe air.

The motor may include one or more air inlets through which cooling airenters the motor, and one or more cooling channels (e.g., passages orconduits) arranged to direct the flow of cooling air towards the statorcoils. In one example, the motor includes a plurality ofcircumferentially arranged cooling channels arranged to direct the flowof cooling air radially outward towards the stator coils. The coolingair may flow in a radial direction through circumferential spacesdefined between circumferentially adjacent flux guiding stator elements.

The motor may include an air accelerating mechanism or device (e.g., anair accelerator) for generating and/or accelerating the flow of coolingair. The accelerator may be or include a fan that may be driven by themotor.

Where direct air cooling is used, each current carrying coil may includean effective cooling surface area that is directly exposed to the flowof cooling air. The directly exposed surface area of the coil may be atleast 20% of a total surface area of the coil. In other words, at least20% of the total surface area of the conductor that forms the coil maybe directly exposed to the flow of cooling air. The effective coolingsurface area may be at least 25%, at least 35%, at least 40%, or atleast 50% of the total surface area of the coil. The effective coolingsurface area may be less than or equal to 90%, less than or equal to80%, less than or equal to 70%, or less than or equal to 60% of thetotal surface area of the coil. The effective cooling surface area maybe in a range of 20% to 80% of the total surface area of the coil, in arange of 25% to 80% of the total surface area of the coil, in a range of30% to 70% of the total surface area of the coil, in a range of 35% to65% of the total surface area of the coil, or in a range of 40% to 60%of the total surface area of the coil. In a specific example, theeffective cooling surface area is in a range of 25% to 45% of the totalsurface area of the coil.

Where a directly cooled electrical machine is used, the aircraft (e.g.,an EPU of the aircraft) may include an air inlet for receiving airproximate to the aircraft for supplying the flow of cooling air to thecoils.

In another group of embodiments, the electrical machine is a radial fluxelectrical machine.

The stator and the rotor of the radial flux electrical machine definemagnetic circuits along which magnetic flux flows. The magnetic fluxpaths may be two-dimensional and lie in a plane perpendicular to an axisof rotation of the electrical machine.

The stator of the radial flux electrical machine may include a pluralityof circumferentially distributed and radially extending stator teethdefining slots therebetween. Each stator coil may be wound around atooth so as to occupy two circumferentially adjacent slots (e.g., aconcentrated winding arrangement). In another example, each stator coilis wound around more than one (e.g., two) teeth so as to occupy morethan one slot (e.g., a distributed winding arrangement).

The rotor of the radial flux electrical machine may include a pluralityof permanent magnets circumferentially distributed about the rotor. Thepermanent magnets of the rotor may face and be separated from the statorby an air gap. The air gap may be a radial air gap (e.g., the rotormagnets may be radially spaced from the stator).

The radial flux electrical machine may be a multi-lane electricalmachine. In other words, the electrical machine may include at least two(e.g., two or four) sub-machines, each sub-machine having anelectrically independent set of stator coils. For example, theelectrical machine may have a first three-phase sub-machine and a secondthree-phase sub-machine.

In one example, the multi-lane radial flux electrical machine has afirst sub-machine having a first stator and a first rotor arranged tointeract with the first stator, and a second sub-machine having a secondstator and a second rotor arranged to interact with the second stator.Axes of rotation of the first rotor and the second rotor are coaxial,but the first sub-machine and the second sub-machine are axially spacedapart from each other. The first rotor and the second rotor may bemechanically coupled so that the first rotor and the second rotor rotatetogether.

In another example, a stator includes a first set of stator teeth andassociated stator coils belonging to a first sub-machine, and the statorincludes a second set of stator teeth and associated stator coilsbelonging to a second sub-machine. The first set of stator teeth and thesecond set of stator teeth and coils are arranged to interact with acommon rotor.

In yet another example, the multi-lane radial flux electrical machinehas at least four lanes (e.g., at least four sub-machines). Theelectrical machine has a first stator and a corresponding first rotor,and a second stator and a corresponding second rotor. Axes of rotationof the first rotor and the second rotor are coaxial, but the firststator and the first rotor are axially spaced apart from the secondstator and the second rotor. The first rotor and the second rotor may bemechanically coupled so that the first rotor and the second rotor rotatetogether. The first stator includes a first set of stator teeth andassociated stator coils belonging to a first sub-machine, and the firststator includes a second set of stator teeth and associated stator coilsbelonging to a second sub-machine. The first set of stator teeth, thesecond set of stator teeth, and stator coils are arranged to interactwith the first rotor. The second stator includes a third set of statorteeth and associated stator coils belonging to a third sub-machine, andthe second stator includes a fourth set of stator teeth and associatedstator coils belonging to a fourth sub-machine. The third set of statorteeth, the fourth set of stator teeth, and stator coils are arranged tointeract with the second rotor.

The following may be applied to in any of the above aspects, singularlyand, except where mutually exclusive, in combination.

The electrical machine may be a motor and may be configured to produce apeak rated torque of τ_(peak) and a maximum continuous rated torque ofτ_(max,cont) Those skilled in the art will understand that the peakrated torque is the highest torque the motor is rated to produce forshort periods (e.g., for transients). For example, τ_(peak) may be thehighest torque the motor can produce for three seconds at ISA sea levelconditions. Sustained operation at the peak rated torque is not possibleand will result in, for example, overheating and damage to the motor. Incontrast, the maximum continuous rated torque is the highest torque themotor can produce and sustain at ISA sea level conditions withoutexceeding a rated temperature of the motor. For example, τ_(max,cont)may be the highest torque the motor can produce for at least threeminutes at ISA sea level conditions.

The electrical machine has an active parts mass, m_(act). The activeparts mass is a cumulated (e.g., total) mass of components of theelectrical machine that contribute to producing the torque (or,equivalently, generating electrical power where the electrical machineis configured as a generator). The active parts mass, matt, includes anyflux guiding material included in the stator and/or the rotor. Thismaterial may be referred to as “iron,” though those skilled in the artwill appreciate that the stator iron and/or rotor iron is not elementaliron in many examples (e.g., the iron may include laminations of aferromagnetic material such as CoFe). The active parts mass, m_(act),also includes the mass of the stator coils. Herein, the mass of thestator coils includes the mass of end windings of the stator coils andthe mass of insulating material surrounding the conductor that forms thestator coils. Although the end windings and the insulating material donot add to the torque produced by the motor, zero torque would beproduced in their absence and so their mass is included in the activeparts mass. The active parts mass, m_(act), also includes the mass ofany flux generating components of the rotor. In a permanent magnetelectric machine, this is the permanent magnets of the rotor. If therotor includes current-carrying coils, these are included in the activeparts mass.

An active parts torque density of the electrical machine, p_(act), isdefined as a ratio of the peak rated torque and the active parts mass:

$\begin{matrix}{\rho_{act} = \frac{\tau_{peak}}{m_{act}}} & (1)\end{matrix}$

According to the present disclosure, a value of p_(act) may be greaterthan or equal to 50 Nmkg⁻¹ (50 Newton meters per kilogram). For example,p_(act) may be in the range of 50 to 165 Nmkg⁻¹.

The active parts torque density, p_(act) may be greater than or equal to55 Nmkg⁻¹, greater than or equal to 60 Nmkg⁻¹, greater than or equal to65 Nmkg⁻¹, greater than or equal to 70 Nmkg⁻¹, greater than or equal to75 Nmkg⁻¹, greater than or equal to 80 Nmkg⁻¹, greater than or equal to85 Nmkg⁻¹, or greater than or equal to 90 Nmkg⁻¹. P_(act) may be lessthan or equal to 160 Nmkg⁻¹, less than or equal to 150 Nmkg⁻¹, less thanor equal to 140 Nmkg⁻¹, less than or equal to 130 Nmkg⁻¹, or less thanor equal to 120 Nmkg⁻¹. p_(act) may be in the range of 60 to 150 Nmkg⁻¹,in the range of 70 to 140 Nmkg⁻¹, in the range of 75 to 130 Nmkg⁻¹, orin the range of 80 to 120 Nmkg⁻¹. In a specific example, the activeparts torque density, p_(act), is in the range of 90 to 110 Nm kg⁻¹.

The electrical machine may further include a cooling system for removingheat from the electrical machine. The cooling system may have a coolingsystem mass, m_(cool) The cooling system mass, m_(cool), is a cumulatedmass of components of the electrical machine that contribute to coolingthe stator and/or rotor of the electrical machine. The componentsincluded in this mass depend on the type and design of the coolingsystem. For a liquid-cooled electrical machine (e.g., an oil-cooledmachine), the cooling system mass includes the mass of the coolant, themass of the tank and conduits (e.g., piping) that contain the coolant,the mass of the pump(s) that circulate the coolant, and the mass of anyheat exchanger(s) included in the cooling system. The cooling systemmass also includes the mass of additional components such as filters andvalves, if present. For an air-cooled electrical machine, the coolingsystem may, for example, include the mass of one or more air filters,one or more flow guiding mechanisms or devices (e.g., air ducts orchannels), and/or one or more structurally integrated fans. For anindirectly air-cooled electrical machine, the cooling system mass mayfurther include the mass of one or more heat exchangers.

A torque density parameter p_(act+cool) may be defined as:

$\begin{matrix}{\rho_{{act} + {cool}} = \frac{\tau_{peak}}{m_{act} + m_{cool}}} & (2)\end{matrix}$

According to the present disclosure, a value of p_(act+cool) may begreater than or equal to 40 Nmkg⁻¹ (e.g., p_(act+cool) may be in therange of 40 to 150 Nmkg⁻¹).

The torque density parameter p_(act+cool) may be greater than or equalto 45 Nmkg⁻¹, greater than or equal to 50 Nmkg⁻¹, greater than or equalto 55 Nmkg⁻¹, greater than or equal to 65 Nmkg⁻¹, or greater than orequal to 70 Nmkg⁻¹. p_(act+cool) may be less than or equal to 140Nmkg⁻¹, less than or equal to 130 Nmkg⁻¹, less than or equal to 120Nmkg⁻¹, less than or equal to 110 Nmkg⁻¹, less than or equal to 100Nmkg⁻¹, or less than or equal to 90 Nmkg⁻¹. p_(act+cool) may be in therange of 45 to 130 Nmkg⁻¹, in the range of 55 to 120 Nmkg⁻¹, in therange of 60 to 110 Nmkg⁻¹, or in the range of 65 to 95 Nmkg⁻¹. In aspecific example, the active parts torque density, P_(act+cool), is inthe range of 70 to 85 Nmkg⁻¹.

The stator of the electrical machine may include flux guiding statoriron (e.g., flux guiding stator elements) defining one or more statorslots that house the stator coils. When producing the peak rated torque,τ_(peak), a slot current density of each slot may be equal toJ_(slot,peak).

A machine parameter Λ may be defined as:

$\begin{matrix}{\Lambda = \frac{\tau_{peak}}{m_{act} \times J_{{s{lot}},{peak}}}} & (3)\end{matrix}$

According to the present disclosure, a value of Λ may be greater than orequal to 5 μNm³ kg⁻¹A⁻¹ (5×10⁻⁶ Newton meters-cubed per kilogram perAmpere). For example, Λ may be in the range of 5 to 35 μNm³ kg⁻¹A⁻¹.

The value of Λ may be greater than or equal to 6 μNm³ kg⁻¹A⁻¹, greaterthan or equal to 7 μNm³ kg⁻¹A⁻¹, greater than or equal to 8 μNm³kg⁻¹A⁻¹, greater than or equal to 9 μNm³ kg⁻¹A⁻¹, greater than or equalto 10 μNm³ kg⁻¹A⁻¹, greater than or equal to 11 μNm³ kg⁻¹A⁻¹, or greaterthan or equal to 12 μNm³ kg⁻¹A⁻¹. The value of A may be less than orequal to 30 μNm³ kg⁻¹A⁻¹, less than or equal to 25 μNm³ kg⁻¹A⁻¹, lessthan or equal to 20 μNm³ kg⁻¹A⁻¹, or less than or equal to 15 μNm³kg⁻¹A⁻¹. The value of A may be in the range of 6 to 22 μNm³ kg⁻¹A⁻¹, inthe range of 7 to 21 μNm³ kg⁻¹A⁻¹, in the range of 8 to 20 μNm³ kg⁻¹A⁻¹,in the range of 9 to 19 μNm³ kg⁻¹A⁻¹ or in the range of 10 to 18 μNm³kg⁻¹A⁻¹. In a specific example, Λ may be in the range of 11 to 17 μNm³kg⁻¹A⁻¹.

Where the electrical machine has a cooling system with cooling systemmass m_(cool), a machine parameter Λ* may be defined as:

$\begin{matrix}{\Lambda^{*} = \frac{\tau_{peak}}{( {m_{act} + m_{cool}} ) \times J_{{slot},{peak}}}} & (4)\end{matrix}$

According to the present disclosure, a value of Λ* may be greater thanor equal to 4 μNm³ kg⁻¹A⁻¹ (e.g., Λ* may be in the range of 4 to 25 μNm³kg⁻¹A⁻¹).

The value of Λ* may be greater than or equal to 5 μNm³ kg⁻¹A⁻¹, greaterthan or equal to 6 μNm³ kg⁻¹A⁻¹, greater than or equal to 7 μNm³kg⁻¹A⁻¹, greater than or equal to 8 μNm³ kg⁻¹A⁻¹, greater than or equalto 9 μNm³ kg⁻¹A⁻¹¹, or greater than or equal to 10 μNm³ kg⁻¹A⁻¹. Thevalue of Λ* may be less than or equal to 20 μNm³ kg⁻¹A⁻¹, less than orequal to 17 μNm³ kg⁻¹A⁻¹, less than or equal to 15 μNm³ kg⁻¹A⁻¹, or lessthan or equal to 13 μNm³ kg⁻¹A⁻¹. The value of Λ* may be in the range of5 to 20 μNm³ kg⁻¹A⁻¹, in the range of 6 to 19 μNm³ kg⁻¹A⁻¹, in the rangeof 7 to 17 μNm³ kg⁻¹A⁻¹, or in the range of 8 to 15 μNm³ kg⁻¹A⁻¹. In aspecific example, Λ* may be in the range of 9 to 12 μNm³ kg⁻¹A⁻¹.

The active parts torque density, p_(act), may be in the range of 50 to165 Nmkg⁻¹, while the slot current density, J_(slot,peak), may be in therange of 3 to 11 A(mm)⁻². p_(act) may in the range of 60 to 140 Nmkg⁻¹,while J_(slot,peak) may be in the range of 4 to 10 A(mm)⁻². P_(act) maybe in the range of 70 to 130 Nmkg⁻¹, while J_(slot,peak) may be in therange of 5 to 9 A(mm)⁻². p_(act) may be in the range of 80 to 120Nmkg⁻¹, while J_(slot,peak) may be in the range of 6 to 8 A(mm)⁻².Herein, the unit “A(mm)⁻²” is “Amperes per square millimeter” (i.e.,10⁶×Amperes per square meter).

The stator coils include an electrically conductive material, and acumulated volume of the conductor material is equal to V_(conductor) Thestator and/or rotor may include iron material configured to guidemagnetic flux in magnetic circuits through the rotor and the stator, anda cumulated volume of the iron material of the stator and the rotor isequal to V_(iron).

A dimensionless machine parameter Γ may be defined as:

$\begin{matrix}{\Gamma = \frac{V_{conductor}}{V_{iron}}} & (5)\end{matrix}$

According to the present disclosure, the value of Γ may be greater thanor equal to 0.25 (e.g., Γ may be in the range 0.25 to 3).

The value of Γ may be greater than or equal to 0.3, greater than orequal to 0.35, greater than or equal to 0.4, greater than or equal to0.45, greater than or equal to 0.5, greater than or equal to 0.55, orgreater than or equal to 0.6. The value of r may be less than or equalto 3.0, less than or equal to 2.5, less than or equal to 2, less than orequal to 1.5, less than or equal to 1, or less than or equal to 0.75. Γmay be in the range of 0.3 to 2.0, in the range of 0.3 to 1.0, in therange of 0.35 to 1.0, in the range of 0.35 to 0.9, in the range of 0.4to 0.8, or in the range of 0.45 to 0.75. In a specific example, Γ is inthe range of 0.5 to 0.7.

Where the electrical machine is a multi-lane electrical machine, theconductor volume V_(conductor) is the cumulated volume of the statorcoils of all sub-machines (e.g., the first and the second sub-machinesof a dual-lane machine). Likewise, V_(iron) is the cumulated mass ofiron material of the rotor and stator of all sub-machines.

The electrical machine has a power factor equal to cos(ϕ), ϕ being asteady-state phase difference between stator coil current and a statorcoil voltage.

A machine parameter Δ may be defined as:

$\begin{matrix}{\Delta = \frac{\rho_{act}}{\cos(\varnothing)}} & (6)\end{matrix}$

According to the present disclosure, a value of Δ may be greater than orequal to 65 Nmkg⁻¹ (e.g., Δ may be in the range 65 to 275 Nmkg⁻¹).

The value of Δ may be greater than or equal to 75 Nmkg⁻¹, greater thanor equal to 85 Nmkg⁻¹, greater than or equal to 95 Nmkg⁻¹, greater thanor equal to 105 Nmkg⁻¹, greater than or equal to 115 Nmkg⁻¹, or greaterthan or equal to 125 Nmkg⁻¹. The value of Δ may be less than or equal to275 Nmkg⁻¹, less than or equal to 250 Nmkg⁻¹, less than or equal to 225Nmkg⁻¹, less than or equal to 200 Nmkg⁻¹, less than or equal to 175Nmkg⁻¹, or less than or equal to 150 Nmkg⁻¹. Δ may be in the range of 70to 200 Nmkg⁻¹, in the range of 75 to 200 Nmkg⁻¹, in the range of 80 to190 Nmkg⁻¹, in the range of 90 to 180 Nmkg⁻¹, in the range of 100 to 170Nmkg⁻¹, or in the range of 110 to 160 Nmkg⁻¹. In a specific example, Δis in the range of 130 to 150 Nmkg⁻¹.

Where the electrical machine has a cooling system with cooling systemmass m_(cool), a machine parameter Δ* may be defined as:

$\begin{matrix}{\Delta^{*} = \frac{\tau_{peak}}{( {m_{act} + m_{cool}} ) \times {\cos(\varnothing)}}} & (7)\end{matrix}$

According to the present disclosure, a value of Δ* may be greater thanor equal to 50 Nmkg⁻¹. For example, Δ* may be in the range of 50 to 190Nmkg⁻¹.

The value of Λ* may be greater than or equal to 60 Nmkg⁻¹, greater thanor equal to 70 Nmkg⁻¹, greater than or equal to 80 Nmkg⁻¹, greater thanor equal to 90 Nmkg⁻¹, or greater than or equal to 100 Nmkg⁻¹. The valueof Δ* may be less than or equal to 175 Nmkg⁻¹, less than or equal to 150Nmkg⁻¹, less than or equal to 140 Nmkg⁻¹, less than or equal to 130Nmkg⁻¹, or less than or equal to 120 Nmkg⁻¹. Δ* may be in the range of55 to 195 Nmkg⁻¹, in the range of 75 to 145 Nmkg⁻¹, in the range of 80to 140 Nmkg⁻¹, in the range of 85 to 135 Nmkg⁻¹, or in the range of 90to 130 Nmkg⁻¹. In a specific example, Λ* is in the range of 100 to 120Nmkg⁻¹.

Those skilled in the art will appreciate that the power factor, cos(ϕ),of an electrical machine may alternatively be defined as a ratio of themain magnetic flux and the total magnetic flux. The total magnetic fluxis the sum of the main magnetic flux and the leakage flux. Specifically:

$\begin{matrix}{{\cos(\varnothing)} = {\frac{{Main}{Flux}}{{{Main}{Flux}} + {Le{akage}{Flux}}} = \frac{{{Total}{Flux}} - {Le{akage}{Flux}}}{{Total}{Flux}}}} & (8)\end{matrix}$

Where the electrical machine is a permanent magnet synchronouselectrical machine, the rotor includes a plurality of circumferentiallydistributed permanent magnets forming a number, N_(P), of rotor poles.The rotor poles have a pole pitch angle, P_(θ), equal to 27 divided bythe number of poles, N_(p). Equivalently, the pole pitch angle, P_(θ),is equal to π divided by the number of pole pairs, N_(Pairs). Thepermanent magnets rotor poles further define a pole arc length, P_(L),equal to a length of an arc at the active parts diameter, D_(act), ofthe electrical machine corresponding to one pole. The active partsdiameter is a diameter corresponding to a radially outermost componentof the electrical machine that contributes to producing the torque (orcontributes to generating the electrical power if the machine isconfigured to operate as an electric generator).

The electrical machine has an air gap separating the rotor from thestator and having an air gap distance G_(Air). In some examples, the airgap is a radial air gap (e.g., the air gap distance is definedperpendicular to an axis of rotation of the rotor of the electricalmachine). In other examples, the air gap is an axial air gap (e.g., theair gap distance is defined parallel to the axis of rotation of therotor of the electrical machine).

A machine parameter γ may be defined as:Y=P _(θ) ×G _(Air)  (9)

According to the present disclosure, a value of γ may be less than orequal to 100 micro radian-meters. For example, γ may be in the range of5 to 100 micro radian-meters.

The value of γ may be less than or equal to 90 micro radian-meters, lessthan or equal to 80 micro radian-meters, less than or equal to 70 microradian-meters, less than or equal to 60 micro radian-meters, less thanor equal to 50 micro radian-meters, less than or equal to 40 microradian-meters, or less than or equal to 30 micro radian-meters. Thevalue of γ may be greater than or equal to 6 micro radian-meters,greater than or equal to 8 micro radian-meters, greater than or equal to10 micro radian-meters, greater than or equal to 12 micro radian-meters,greater than or equal to 15 micro radian-meters, or greater than orequal to 18 micro radian-meters. γ may be in the range of 7 to 90 microradian-meters, in the range of 9 to 75 micro radian-meters, in the rangeof 11 to 60 micro radian-meters, or in the range of 12 to 40 microradian-meters. In a specific example, γ is in the range of 15 to 30micro radian-meters.

In a specific example, the electrical machine is a transverse fluxelectrical machine with a dual rotor. The dual rotor includes a firstrotor portion and a second rotor portion spaced apart from the firstrotor portion. The stator is located between the first rotor portion andthe second rotor portion. The first rotor portion has a first pluralityof permanent magnets distributed about a circumference of the firstrotor portion, the permanent magnets of the first plurality formingN_(p) rotor poles having a pole pitch angle P_(θ). The second rotorportion has a second plurality of permanent magnets distributed about acircumference of the second rotor portion, the permanent magnets of thesecond plurality forming N_(p) rotor poles having a pole pitch angleP_(θ). A first air gap separates the first rotor portion from a firstside of the stator by a first air gap distance G_(Air,1). A second airgap separates the second rotor portion from a second side of the statorby a second air gap distance G_(Air,2) In this example, for each of thefirst air gap and the second air gap, the machine parameter γ is lessthan or equal to 100 micro radian-meters.

A machine parameter γ may be defined as:γ*=P _(L) ×G _(Air)  (10)

According to the present disclosure, a value of γ may be less than orequal to 40 μm² (40×10⁻⁶ square-meters). For example, Y*may be in therange of 1 to 40 μm².

The value of γ* may be less than or equal to 35 μm², less than or equalto 30 μm², less than or equal to 25 μm², less than or equal to 20 μm²,less than or equal to 15 μm², or less than or equal to 10 μm². The valueof γ* may be greater than or equal to 1.5 μm², greater than or equal to2 μm², or greater than or equal to 2.5 μm². γ* may be in the range 1.5to 30 μm², in the range 2 to 20 μm², in the range 2.5 to 15, or in therange 3 to 10 μm². In a specific example, γ is in the range 3.5 to 7.5μm².

If configured as a motor, the electrical machine may, in use, beconfigured to receive current from a DC:AC power electronics converter(e.g., an inverter). The maximum frequency of the current receivedduring use of the electrical machine may be equal to f_(max). The term“maximum frequency of the current” refers to the highest value of thefundamental frequency of the current received during use, and not to themaximum frequency of a harmonic component of the current.

A machine parameter Π may be defined as:

$\begin{matrix}{\Pi = \frac{P_{L}}{f_{\max}}} & (11)\end{matrix}$

According to the present disclosure, a value of Π may be less than orequal to 30 μms (30×10⁻⁶ meter-seconds). For example, Π may be in therange of 1 to 30 μms.

The value of Π may be less than or equal to 25 μms, less than or equalto 20 μms, less than or equal to 15 μms, less than or equal to 10 μms,or less than or equal to 7.5 μms. Π may be greater than or equal to 1μms, greater than or equal 1.5 μms, greater than or equal to 2.0 μms,greater than or equal to 2.5 μms, greater than or equal to 3.0 μms, orgreater than or equal to 3.5 μms. The value of Π may be in the range of1.5 to 20 μms, in the range of 2.0 to 15 μms, in the range of 2.5 to 12μms, or in the range of 3.0 to 9 μms. In a specific example, the valueof Π is in the range of 3.5 to 7.5 μms.

A steady-state mechanical speed of rotation of the rotor when the statorcoils are receiving current at the maximum electrical frequency,f_(max), may be less than or equal to 1,500 rpm. The speed may be lessthan or equal to 1,400 rpm, less than or equal to 1,300 rpm, less thanor equal to 1,200 rpm, less than or equal to 1,100 rpm, less than orequal to 1,000 rpm, or less than or equal to 900 rpm. The speed may begreater than or equal to 500 rpm, greater than or equal to 600 rpm, orgreater than or equal to 700 rpm. In other examples, the speed may be ina range of 500 to 1,500 rpm, in a range of 600 to 1,400 rpm, in a rangeof 700 to 1,300 rpm, or in a range of 900 to 1,200 rpm.

As noted previously, the electrical machine may have a cooling systemconfigured to supply, in use, a flow of coolant to remove heat from theelectrical machine (e.g., from the stator coils and, optionally, therotor). The coolant has a specific heat capacity of C_(p) at ISA sealevel conditions. The coolant may be a liquid (e.g., an oil) or air(e.g., ambient air). The coolant may be supplied at a mass flow rate of{dot over (m)}_(coolant) (measured in kgs⁻¹) that may vary duringoperation. A cooling rate, C_(cocl), that may be referred to as the heatcapacity cooling rate (with units of Js⁻¹K⁻¹) is defined as a product ofthe coolant heat capacity C_(p) and the mass flow rate {dot over(m)}_(coolant).

The cooling system may be configured to supply the flow of coolant at amass flow rate of at least {dot over (m)}_(coolant)={dot over(m)}_(max,cont) when the electrical machine is producing the maximumcontinuous rated torque, T_(max,cont) The mass flow rate {dot over(m)}_(max,cont) is the minimum mass flow rate of the coolant, which hasspecific heat capacity C_(p) at ISA sea level conditions, required tomaintain the insulation of the stator coils at or below a maximum ratedinsulation temperature θ_(ins,max).

The maximum continuous rated torque T_(max,cont) may be greater than orequal to 650 Nm and yet a cooling rate C_(max,cont), defined as theproduct of the coolant specific heat capacity, C_(p), at ISA sea levelconditions and the mass flow rate, {dot over (m)}_(max,cont), may beless than or equal to 680 Js⁻¹K⁻¹. Additionally, or alternatively, aratio defined as the maximum continuous rated torque T_(max,cont)divided by the cooling rate C_(max,cont) may be greater than or equal to2 sK (e.g., in the range 2 to 10 sK).

A machine parameter ∇ may be defined as:

$\begin{matrix}{\nabla = \frac{\tau_{\max,{cont}}}{m_{act} \times C_{\max,{cont}}}} & (12)\end{matrix}$

According to the present disclosure, a value of ∇ may be greater than orequal to 0.1 Kskg⁻¹ (Kelvin-seconds per kilogram) (e.g., ∇ may be in therange of 0.1 to 0.8 Kskg⁻¹).

The value of ∇ may be greater than or equal to 0.15 Kskg⁻¹, greater thanor equal to 0.18 Kskg⁻¹, greater than or equal to 0.20 Kskg⁻¹, greaterthan or equal to 0.22 Kskg⁻¹, greater than or equal to 0.25 Kskg⁻¹, orgreater than or equal to 0.30 Kskg⁻¹. The value of ∇ may be less than orequal to 0.7 Kskg⁻¹, less than or equal to 0.65 Kskg⁻¹, less than orequal to 0.6 Kskg⁻¹, less than or equal to 0.5 Kskg⁻¹, or less than orequal to 0.4 Kskg⁻¹. ∇ may be in the range of 0.11 to 0.7 Kskg⁻¹, in therange of 0.14 to 0.65 Kskg⁻¹, or in the range of 0.18 to 0.4 Kskg⁻¹. Ina specific example, Vis in the range of 0.22 to 0.35 Kskg⁻¹.

A machine parameter ∇* may be defined as:

$\begin{matrix}{\nabla^{*} = \frac{\tau_{\max,{cont}}}{( {m_{act} + m_{cool}} ) \times C_{\max,{cont}}}} & (13)\end{matrix}$

According to the present disclosure, a value of ∇* may be greater thanor equal to 0.08 Kskg⁻¹ (e.g., ∇* may be in the range of 0.08 to 0.7Kskg⁻¹).

The value of ∇* may be greater than or equal to 0.10 Kskg⁻¹, greaterthan or equal to 0.12 Kskg⁻¹, greater than or equal to 0.14 Kskg⁻¹,greater than or equal to 0.15 Kskg⁻¹, greater than or equal to 0.16Kskg⁻¹, or greater than or equal to 0.17 Kskg⁻¹. The value of ∇* may beless than or equal to 0.6 Kskg⁻¹, less than or equal to 0.55 Kskg⁻¹,less than or equal to 0.5 Kskg⁻¹, less than or equal to 0.4 Kskg⁻¹, orless than or equal to 0.3 Kskg⁻¹. ∇* may be in the range of 0.11 to 0.55Kskg⁻¹, in the range of 0.13 to 0.45 Kskg⁻¹, or in the range of 0.15 to0.3 Kskg⁻¹. In a specific example, ∇* is in the range of 0.17 to 0.27Kskg⁻¹.

The electrical machine may have an efficiency of η while producing themaximum continuous rated torque, τ_(max,cont) at ISA sea levelconditions.

A machine parameter Z may be defined as:

$\begin{matrix}{Z = \frac{{\cos(\varnothing)} \times m_{act}}{\eta}} & (14)\end{matrix}$

According to the present disclosure, a value of Z may be less than orequal to 30 kg. For example, Z may be in the range of 5 to 30 kg.

The value of Z may be less than or equal to 25 kg, less than or equal to20 kg, less than or equal to 17 kg, less than or equal to 15 kg, or lessthan or equal to 13 kg. The value of Z may be greater than or equal to 7kg, greater than or equal to 8 kg, greater than or equal to 9 kg, orgreater than or equal to 10 kg. The value of Z may be in the range of 7to 25 kg, in the range of 7.5 to 20 kg, in the range of 8 to 17 kg, inthe range of 8.5 to 15 kg, or in the range of 9 to 14 kg. In a specificexample, the value of Z is in the range of 9.5 to 13.5 kg.

A machine parameter Z* may be defined as:

$\begin{matrix}{Z^{*} = \frac{{\cos(\varnothing)} \times ( {m_{act} + m_{cool}} )}{\eta}} & (15)\end{matrix}$

According to the present disclosure, a value of Z* may be less than orequal to 35 kg. For example, Z* may be in the range of 6 to 35 kg.

The value of Z* may be less than or equal to 30 kg, less than or equalto 25 kg, less than or equal to 20 kg, less than or equal to 19 kg, orless than or equal to 17 kg. The value of Z* may be greater than orequal to 8 kg, greater than or equal to 9 kg, greater than or equal to10 kg, or greater than or equal to 11 kg. The value of Z* may be in therange of 8 to 22 kg, in the range of 9 to 19 kg, in the range of 10 to18 kg, in the range of 10.5 to 17.5 kg, or in the range of 11 to 17 kg.In a specific example, the value of Z* is in the range of 11.5 to 16.5kg.

The electrical machine may carry a continuous rated current of I_(cont)when producing the maximum continuous rated torque, τ_(max,cont) Theelectrical machine may carry a peak rated current I_(peak) whenproducing the peak rated torque, τ_(peak) As used herein, the term “peakrated current” refers to an RMS current corresponding to the peak torqueand not to a maximum value of an AC sine wave.

The electrical machine may carry a steady-state current of I_(sc) whensubject to a steady-state terminal short circuit condition. In otherwords, when terminals of the electrical machine are short circuited,following an initial transient, the current carried by the stator coilssettles to I_(sc).

A dimensionless machine parameter ζ may be defined as:

$\begin{matrix}{\xi = \frac{I_{SC}}{I_{peak}}} & (16)\end{matrix}$

According to the present disclosure, the value of ζ may be in the rangeof 0.5 to 1.2.

The value of ζ may be less than or equal to 1.2, less than or equal to1.1, less than or equal to 1.0, or less than or equal to 0.95. The valueof ζ may be greater than or equal to 0.55, greater than or equal to 0.6,or greater than or equal to 0.65. ζmay be in the range of 0.6 to 1.1, inthe range of 0.7 to 0.95, or in the range of 0.65 to 1.0. In a specificexample, is in the range of 0.7 to 0.9.

The stator coils may be insulated. A maximum temperature of theinsulation when carrying the maximum continuous rated current I_(cont)at ISA sea level conditions is equal to θ_(ins)(I_(cont)). A maximumtemperature of the insulation when carrying the steady-state terminalshort circuit current at ISA sea level conditions may be equal toθ_(ins)(I_(sc)). θ_(ins)(I_(cont)) may be less than or equal to themaximum rated temperature of the insulation, θ_(ins,max).

A dimensionless machine parameter ζ may be defined as:

$\begin{matrix}{\zeta = \frac{\theta_{ins}( I_{SC} )}{\theta_{ins}( I_{cont} )}} & (17)\end{matrix}$

According to the present disclosure, a value of ζ may be less than orequal to 1.3. For example, ζ may be in the range of 0.5 to 1.3.

The value of ζ may be less than or equal to 1.2, less than or equal to1.1, less than or equal to 1.0, less than or equal to 0.95, less than orequal to 0.9, or less than or equal to 0.85. The value of ζ may begreater than or equal to 0.55, greater than or equal to 0.6, greaterthan or equal to 0.65, greater than or equal to 0.7, or greater than orequal to 0.75. ζ may be in the range of 0.6 to 1.2, in the range of 0.7to 1.1, or in the range of 0.8 to 1.0. In a specific example, ζ is inthe range of 0.85 to 0.95.

The electrical machine has an inductance equal to L_(machine) Theinductance L_(machine) may be determined by measuring the currentresponse to an AC voltage excitation using the equation:

$\begin{matrix}{V = {L_{machine} \times \frac{dI}{dt}}} & (18)\end{matrix}$

A machine parameter β may be defined as:β=L _(machine) ×P _(act)  (19)

According to the present disclosure, a value of β may be greater than orequal to 1.5 mHNmkg⁻¹ (1.5 milli Henry Newton meters per kilogram). Forexample, β may be in the range of 1.5 to 15 mHNmkg⁻¹.

The value of β may be greater than or equal to 1.6 mHNmkg⁻¹, greaterthan or equal to 1.8 mHNmkg⁻¹, greater than or equal to 2.0 mHNmkg⁻¹,greater than or equal to 2.2 mHNmkg⁻¹, greater than or equal to 2.4mHNmkg⁻¹, greater than or equal to 2.6 mHNmkg⁻¹, greater than or equalto 3.0 mHNmkg⁻¹, or greater than or equal to 3.5 mHNmkg⁻¹. The value ofβ may be less than or equal to 12 mHNmkg⁻¹, less than or equal to 10mHNmkg⁻¹, less than or equal to 8 mHNmkg⁻¹, or less than or equal to 6mHNmkg⁻¹. β may be in the range of 1.7 to 8 mHNmkg⁻¹, in the range of2.1 and 7 mHNmkg⁻¹, or in the range of 2.4 to 6.5 mHNmkg⁻¹. In aspecific example, is in the range of 2.8 to 5.8 mHNmkg⁻¹.

A machine parameter may be defined as:

$\begin{matrix}{\lambda = \frac{\eta \times L_{machine}}{m_{act}}} & (20)\end{matrix}$

According to the present disclosure, a value of may be greater than orequal to 1.4 μHkg⁻¹ (1.4×10⁻⁶ Henrys per kilogram). For example, A maybe in the range of 1.4 to 7.5 μHkg⁻¹.

The value of λ may be greater than or equal to 1.6 μHkg⁻¹, greater thanor equal to 1.8 μHkg⁻¹, greater than or equal to 2.0 μHkg⁻¹, greaterthan or equal to 2.2 μHkg⁻¹, greater than or equal to 2.4 μHkg⁻¹, orgreater than or equal to 2.6 μHkg⁻¹. The value of λ may be less than orequal to 7.0 μHkg⁻¹, less than or equal to 6.8 μHkg⁻¹, less than orequal to 6.5 μHkg⁻¹, less than or equal to 6.0 μHkg⁻¹, less than orequal to 5.5 μHkg⁻¹, less than or equal to 5.0 μHkg⁻¹, less than orequal to 4.5 μHkg⁻¹, or less than or equal to 4.0 μHkg⁻¹. λ may be inthe range of 1.4 to 6.8 μHkg⁻¹, in the range of 1.6 to 6.0 μHkg⁻¹, inthe range of 1.9 to 5.0 μHkg⁻¹, or in the range of 2.2 to 4.5 μHkg⁻¹. Ina specific example, λ is in the range of 2.4 to 3.8 μHkg⁻¹.

A machine parameter λ* may be defined as:

$\begin{matrix}{\lambda^{*} = \frac{\eta \times L_{machine}}{( {m_{act} + m_{cool}} )}} & (21)\end{matrix}$

According to the present disclosure, a value of λ* may be greater thanor equal to 1.1 μHkg⁻¹. For example, λ* may be in the range of 1.1 to6.5 μHkg⁻¹.

The value of λ* may be greater than or equal to 1.3 μHkg⁻¹, greater thanor equal to 1.5 μHkg⁻¹, greater than or equal to 1.7 μHkg⁻¹, greaterthan or equal to 1.9 μHkg⁻¹, or greater than or equal to 2.1 μHkg⁻¹. Thevalue of λ* may be less than or equal to 6.0 μHkg⁻¹, less than or equalto 5.5 μHkg⁻¹, less than or equal to 5.0 μHkg⁻¹, less than or equal to4.5 μHkg⁻¹, less than or equal to 4.0 μHkg⁻¹, less than or equal to 3.5μHkg⁻¹, or less than or equal to 3.0 μHkg⁻¹. λ* may be in the range of1.1 to 5.3 μHkg⁻¹, in the range of 1.4 to 4.8 μHkg⁻¹, in the range of1.6 to 4.4 μHkg⁻¹, or in the range of 1.8 to 3.2 μHkg⁻¹. In a specificexample, λ* is in the range of 2.0 to 3.0 μHkg⁻¹.

The electrical machine may have an active parts diameter equal toD_(act). The active parts diameter is a diameter corresponding to aradially outermost component of the electrical machine that contributesto producing the torque (or contributes to generating the electricalpower if the machine is configured to operate as an electric generator).

The electrical machine may be configured so that the rotor rotates at aspeed of ω_(mech,cont) when producing the maximum continuous ratedtorque, τ_(max,cont).

A dimensionless figure of merit, F, of the electrical machine may bedefined as:

$\begin{matrix}{F = {\frac{\tau_{\max,{cont}}}{m_{act}}\frac{p_{{air},0}}{C_{p}{{\overset{.}{m}}_{\max,{cont}}( {\theta_{{ins},\max} - \theta_{{air},0}} )}}\frac{2\pi \times D_{ref}}{\omega_{{mech},{cont}}}( \frac{D_{ref}}{D_{act}} )^{2}}} & (22)\end{matrix}$

In the above equation, pairs), p_(air,0), θ_(air,0) and D_(ref) areconstants. Specifically, p_(air,0) is a nominal ambient air pressureequal to 100 kPa, θ_(air,0) is a nominal ambient air temperature equalto 318 Kelvin, and D_(ref) is a nominal active parts diameter equal to0.5 meters. The value of ω_(mech,cont) is measured in radians per second(rads⁻¹).

The electrical machine according to the present disclosure may have avalue of F greater than or equal to 1.9. For example, F may be in therange of 1.9 to 17.

The value of F may be greater than or equal to 2.1, greater than orequal to 2.3, greater than or equal to 2.5, greater than or equal to2.7, greater than or equal to 3.0, greater than or equal to 3.5, greaterthan or equal to 4.0, or greater than or equal to 4.5. The value of Fmay be less than or equal to 17, less than or equal to 15, less than orequal to 13, less than or equal to 11, less than or equal to 9, or lessthan or equal to 7. The value of F may be in the range of 2.3 to 16, inthe range of 2.5 to 13, in the range of 2.7 to 11, in the range of 2.9to 7, or in the range of 3.3 to 6.8. In a specific example, F is in therange of 3.8 to 6.5.

A modified figure of merit, F*, may be defined as:

$\begin{matrix}{F^{*} = {\frac{\tau_{\max,{cont}}}{( {m_{act} + m_{cool}} )}\frac{p_{{air},0}}{C_{p}{{\overset{.}{m}}_{\max,{cont}}( {\theta_{{ins},\max} - \theta_{{air},0}} )}}\frac{2\pi \times D_{ref}}{\omega_{{mech},{cont}}}( \frac{D_{ref}}{D_{act}} )^{2}}} & (23)\end{matrix}$

According to the present disclosure, a value of F* may be greater thanor equal to 1.6. For example, F*may be in the range of 1.6 to 14.

The value of F* may be greater than or equal to 1.8, greater than orequal to 2.0, greater than or equal to 2.2, greater than or equal to2.4, greater than or equal to 2.8, greater than or equal to 3.2, greaterthan or equal to 3.6, or greater than or equal to 3.8. The value ofF*may be less than or equal to 12, less than or equal to 10, less thanor equal to 8, less than or equal to 7, or less than or equal to 5.5.The value of F*may be in the range of 2.0 to 10, in the range of 2.2 to9, in the range of 2.4 to 8, in the range of 2.6 to 7, or in the rangeof 2.8 to 6.0. In a specific example, F* is in the range of 3.0 to 5.5.

In a VTOL aircraft, a take-off parameter x may be defined as:

$\begin{matrix}{\chi = \frac{v_{tip} \times m_{act}}{\tau_{peak}}} & (24)\end{matrix}$

In the above equation, ν_(tip) is the maximum tip speed, measured inms⁻¹, of the propeller or fan of the EPU to occur during a verticaltake-off operation of the VTOL aircraft. This is equal to 27 multipliedby a radius of the propeller or fan of the EPU divided by the mechanicalfrequency (in Hz) of rotation of the propeller or fan. In a directlydriven EPU arrangement, the mechanical frequency of rotation of thepropeller or fan is equal to the mechanical frequency of rotation of therotor of the electrical machine. In an indirectly driven arrangement,the mechanical frequency of rotation of the propeller or fan will bedifferent from (e.g., differ by a gear ratio from) the mechanicalfrequency of rotation of the rotor of the electrical machine.

According to the present disclosure, a value of x may be less than orequal to 7.5 sm⁻¹ (9.0 seconds per meter). For example, x may be in therange of 0.5 to 7.5 sm⁻¹.

The value of x may be less than or equal to 6.5 sm⁻¹, less than or equalto 6.0 sm⁻¹, less than or equal to 5.5 sm⁻¹, less than or equal to 5.0sm⁻¹, less than or equal to 4.5 sm⁻¹, less than or equal to 4.0 sm⁻¹,less than or equal to 3.5 sm⁻¹, less than or equal to 3.0 sm⁻¹, or lessthan or equal to 2.5 sm⁻¹. The value of x may be greater than or equalto 0.75 sm⁻¹, greater than or equal to 1.0 sm⁻¹, greater than or equalto 1.25 sm⁻¹, greater than or equal to 1.5 sm⁻¹, or greater than orequal to 1.75 sm⁻¹. The value of x may be in the range of 0.8 to 4.3sm⁻¹, in the range of 1.2 to 3.8 sm⁻¹, in the range of 1.4 to 3.2 sm⁻¹,or in the range of 1.6 to 2.5 sm⁻¹. In a specific example, Xis in therange of 1.8 and 2.4 sm⁻¹.

A take-off parameter x* may be defined as:

$\begin{matrix}{\chi^{*} = \frac{v_{tip} \times ( {m_{act} + m_{cool}} )}{\tau_{peak}}} & (25)\end{matrix}$

According to the present disclosure, a value of x* may be less than orequal to 9.0 sm⁻¹. For example, x* may be in the range of 1.1 to 9.0sm⁻¹.

The value of x* may be less than or equal to 8.0 sm⁻¹, less than orequal to 7.0 sm⁻¹, less than or equal to 6.0 sm⁻¹, less than or equal to5.5 sm⁻¹, less than or equal to 5.0 sm⁻¹, less than or equal to 4.5sm⁻¹, less than or equal to 4.0 sm⁻¹, less than or equal to 3.5 sm⁻¹, orless than or equal to 3.0 sm⁻¹. The value of x* may be greater than orequal to 1.2 sm⁻¹, greater than or equal to 1.4 sm⁻¹, greater than orequal to 1.6 sm⁻¹, greater than or equal to 1.8 sm⁻¹, or greater than orequal to 2.0 sm⁻¹. The value of x* may be in the range of 1.1 to 5.3sm⁻¹, in the range of 1.4 to 5.1 sm⁻¹, in the range of 1.6 to 4.5 sm⁻¹,or in the range of 1.8 to 4.0 sm⁻¹. In a specific example, X* is in therange of 2.0 to 3.0 sm⁻¹.

In a VTOL aircraft, a hover parameter W may be defined as:

$\begin{matrix}{\Psi = \frac{\tau_{hover}}{\omega_{hover}}} & (26)\end{matrix}$

In the above equation, τ_(hover) is the continuous torque produced bythe motor while the VTOL aircraft is hovering. ω_(hover) is thecontinuous angular speed of rotation of the rotor of the motor while theVTOL aircraft is hovering, measured in radians per second. The term“hovering” refers to a state in which the EPU(s) of the aircraft areproducing sufficient thrust to lift the weight of the VTOL aircraft andmaintain a constant altitude above ground, with substantially no lateralmovement and without requiring airframe (e.g., wing-borne) lift.According to the present disclosure, a value of Ψ may be greater than orequal to 5 Nmsrad⁻¹ (5 Newton meter seconds per radian) (e.g., in arange of 5 to 20 Nmsrad⁻¹). In a group of examples in which the hoverparameter Ψ is in one of the above-described ranges, the VTOL aircraftmay have a direct drive EPU (e.g., with no rotational speed changingmechanisms or devices, such as a transmission) between the motor and thepropeller.

The value of W may be greater than or equal to 6 Nmsrad⁻¹, greater thanor equal to 7 Nmsrad⁻¹, greater than or equal to 8 Nmsrad⁻¹, greaterthan or equal to 9 Nmsrad⁻¹, or greater than or equal to 9.5 Nmsrad⁻¹.The value of Ψ may be less than or equal to 18 Nmsrad⁻¹, less than orequal to 16 Nmsrad⁻¹, less than or equal to 14 Nmsrad⁻¹, less than orequal to 13 Nmsrad⁻¹, or less than or equal to 12 Nmsrad⁻¹. The value ofW may be in the range of 6 to 16.5 Nmsrad⁻¹, in the range of 7 to 15Nmsrad⁻¹, in the range of 8 to 13 Nmsrad⁻¹, or in the range of 9 to 11Nmsrad⁻¹. In a specific example, W is in the range of 9.4 to 10.4Nmsrad⁻¹.

The maximum continuous rated power of the electrical machine, P_(cont),may be in the range of 50 to 400 kW. P_(cont) may be in the range of 60to 350 kW, in the range of 60 to 300 kW, in the range of 75 to 250 kW,in the range of 85 to 225 kW, or in the range of 90 to 175 kW.

The peak rated power of the electrical machine, P_(peak), may be in therange of 60 to 450 kW, in the range of 70 to 400 kW, in the range of 75to 350 kW, in the range of 80 to 300 kW, in the range of 85 to 250 kW,or in the range of 90 to 225 kW.

The maximum continuous rated torque, τ_(max,cont), may be greater thanor equal to 700 Nm, greater than or equal to 750 Nm, in the range of 700to 1,800 Nm, in the range of 800 to 1,700 Nm, in the range of 900 to1,600 Nm, in the range of 1,000 to 1,500 Nm, or in the range of 1,100 to1,450 Nm. In a specific example, τ_(max,cont) is in the range of 1,150to 1,400 Nm.

The peak rated torque, τ_(peak), may be greater than or equal to 800 Nm,in the range of 800 to 2,000 Nm, in the range of 900 to 1,900 Nm, in therange of 1,000 to 1,800 Nm, in the range of 1,100 to 1,700 Nm, or in therange of 1,200 to 1,650 Nm. In a specific example, τ_(peak) is in therange of 1,300 to 1,600 Nm.

A ratio equal to the maximum continuous rated torque, τ_(max,cont),divided by the peak rated torque, τ_(peak), may be greater than or equalto 0.75, greater than or equal to 0.8, greater than or equal to 0.85, orgreater than or equal to 0.9.

The hover torque, τ_(hover), may be in the range of 500 to 1,500 Nm, inthe range of 600 to 1,400 Nm, in the range of 500 to 1,300 Nm, in therange of 700 to 1,200 Nm, or in the range of 750 to 1,150 Nm. In aspecific example, τ_(hover) is in the range of 850 to 1,000 Nm.

The active parts mass, m_(act), may be greater than or equal to 9 kg,greater than or equal to 10 kg, less than or equal to 30 kg, less thanor equal to 25 kg, less than or equal to 22 kg, less than or equal to 20kg, less than or equal to 18 kg, or less than or equal to 16 kg. Theactive parts mass, m_(act), may be in the range of 10 to 30 kg or in therange of 9 to 16 kg.

The cooling system mass, m_(cool), may be less than or equal to 12 kg,less than or equal to 11 kg, less than or equal to 10 kg, less than orequal to 9 kg, less than or equal to 8 kg, less than or equal to 7 kg,less than or equal to 6 kg, or less than or equal to 5 kg. Theelectrical machine may be air cooled.

The slot current density at peak rated torque, J_(slot,peak), may beless than or equal to 15 A(mm)⁻², less than or equal to 14 A(mm)⁻², lessthan or equal to 13 A(mm)⁻², less than or equal to 12 A(mm)⁻², less thanor equal to 11 A(mm)⁻², less than or equal to 10.5 A(mm)⁻², less than orequal to 10 A(mm)⁻², less than or equal to 9.5 A(mm)⁻² less than orequal to 9 A(mm)⁻², less than or equal to 8 A(mm)⁻², or less than orequal to 7 A(mm)⁻². J_(slot,peak) may be greater than or equal to 3A(mm)⁻², greater than or equal to 4 A(mm)⁻² or greater than or equal to3 A(mm)⁻².

The peak current, I_(peak), may be less than or equal to 500 A, lessthan or equal to 450 A, less than or equal to 400 A, less than or equalto 350 A, less than or equal to 330 A, less than or equal to 300 A, lessthan or equal to 270A, or less than or equal to 250 A. I_(peak) may bein the range of 160 A to 400 A, in the range of 170 A to 370 A, in therange of 180 A to 340 A, or in the range of 190 A to 310 A. In oneexample, I_(peak) is in the range of 200 A to 280 A. The values are RMSvalues.

The continuous rated current, I_(cont max) may be less than or equal to300 A, less than or equal to 250 A, less than or equal to 230 A, lessthan or equal to 220 A, or less than or equal to 210 A. I_(cont max) maybe in the range of 130 A to 260 A, in the range of 140 A to 250 A, inthe range of 150 A to 240 A, or in the range of 160 A to 230 A. Thevalues are RMS values.

The steady-state terminal short circuit current, I_(sc), per phase, maybe less than or equal to 270 A, less than or equal to 250 A, less thanor equal to 230 A, less than or equal to 210 A, less than or equal to200 A, or less than or equal to 190 A. I_(sc) may be in the range of 110A to 240 A, in the range of 120 A to 230 A, in the range of 130 A to 220A, or in the range of 165 A to 210 A. The values are RMS values.

The coolant may be air, which has a specific heat capacity, C_(p), ofapproximately 1,006 Jkg⁻¹K⁻¹ at ISA sea level conditions. Alternatively,the coolant may be a liquid (e.g., an oil). In a specific example, thecoolant is a mineral oil with a specific heat capacity of approximately1745 Jkg⁻¹K⁻¹ at ISA sea level conditions.

The coolant mass flow rate, {dot over (m)}_(max,cont), may be at least0.15 kgs⁻¹, at least 0.20 kgs⁻¹, at least 0.25 kgs⁻¹, or at least 0.3kgs⁻¹. The coolant mass flow rate {dot over (m)}_(cont,max) may be lessthan or equal to 2.5 kgs⁻¹, less than or equal to 1.0 kgs⁻¹, less thanor equal to 0.75 kgs⁻¹, or less than or equal to 0.5 kgs⁻¹. The massflow rate may be in the range of 0.15 to 0.50 kgs⁻¹.

The heat capacity cooling rate at maximum continuous rated torque,C_(max,cont) may be less than or equal to 600 Js⁻¹K⁻¹, less than orequal to 500 Js⁻¹K⁻¹, or less than or equal to 480 Js⁻¹K⁻¹. C_(max,cont)may be in the range of 150 to 450 Js⁻¹K⁻¹, in the range of 180 to 420Js⁻¹K⁻¹, in the range of 200 to 400 Js⁻¹K⁻¹, in the range of 220 to 380Js⁻¹K⁻¹, or in the range of 230 to 350 Js⁻¹K⁻¹. In an example,C_(max,cont) is in the range of 260 to 340 Js⁻¹K⁻¹.

The efficiency, η, of the electrical machine when producing thecontinuous rated torque, T_(max,cont), may be at least 90%. Theefficiency may be in the range of 90% to 96% or in the range of 92% to95%.

The conductor volume, V_(conductor), may be in the range of 10 to 100cm³, in the range of 15 to 80 cm³, in the range of 20 to 60 cm³, in therange of 25 to 50 cm³, or in the range of 30 to 40 cm³.

The iron volume, V_(iron), may be in the range of 30 to 150 cm³, in therange of 35 to 120 cm³, in the range of 40 to 90 cm³, in the range of 45to 80 cm³, or in the range of 50 to 70 cm³.

The power factor, cos(ϕ), may be less than or equal to 0.9. The powerfactor, cos(ϕ), may be in the range of 0.6 to 0.9, in the range of 0.6to 0.85, in the range of 0.65 to 0.85, or in the range of 0.65 to 0.8.In a specific example, the power factor, cos(ϕ), is in the range of 0.65to 0.75.

The active parts diameter, D_(act), may be in the range of 0.25 to 1.25meters, in the range of 0.3 to 1.0 meters, in the range of 0.35 to 0.75meters, or in the range of 0.4 to 0.6 meters.

The number of rotor poles, N_(P), may be greater than or equal to 30,greater than or equal to 40, greater than or equal to 50, greater thanor equal to 60, greater than or equal to 70, greater than or equal to80, greater than or equal to 90, greater than or equal to 100, greaterthan or equal to 110, greater than or equal to 120, greater than orequal to 130, greater than or equal to 140, greater than or equal to150, greater than or equal to 160, greater than or equal to 170, greaterthan or equal to 180, greater than or equal to 190, or greater than orequal to 200. The number of rotor poles, N_(P), may be greater than orequal to 250, greater than or equal to 300, greater than or equal to350, or greater than or equal to 400. The number of rotor poles, N_(P),may be less than or equal to 250. The number of rotor poles, N_(P), maybe in in the range of 120 to 250. The pole pair number is equal to halfthe number of rotor poles (i.e., N_(P) divided by two).

The rotor pole pitch, P_(θ), equal to 360° (2Π radians) divided by thenumber of rotor poles, or equally 180° (Π radians) divided by the polepair number, may be less than or equal to 10° (0.174 radians). P_(θ) maybe less than or equal to 9° (0.157 radians), less than or equal to 8°(0.140 radians), less than or equal to 7° (0.122 radians), less than orequal to 6° (0.105 radians), less than or equal to 5° (0.087 radians),less than or equal to 4° (0.070 radians), or less than or equal to 3°(0.052 radians). P_(θ) may be in the range of 1° to 5° (in the range of0.02 to 0.09 radians).

The rotor pole arc length, P_(L), may be less than or equal to 25 mm,less than or equal to 23 mm, less than or equal to 21 mm, less than orequal to 19 mm, less than or equal to 17 mm, less than or equal to 15mm, less than or equal to 13 mm, less than or equal to 12 mm, less thanor equal to 11 mm, less than or equal to 10 mm, less than or equal to 9mm, less than or equal to 8 mm, less than or equal to 7 mm, or less thanor equal to 6 mm. P_(L) may be in the range of 3 to 15 mm, in the rangeof 4 to 12 mm, or in the range of 5 to 10 mm.

The (or each) air gap, G_(Air), may be less than or equal to 2 mm, lessthan or equal to 1.8 mm, less than or equal to 1.6 mm, less than orequal to 1.4 mm, less than or equal to 1.2 mm, less than or equal to 1.0mm, less than or equal to 0.9 mm, less than or equal to 0.8 mm, or lessthan or equal to 0.7 mm. G_(Air) may be in the range of 0.4 to 1.5 mm,in the range of 0.45 to 1.3 mm, in the range of 0.5 to 1.1 mm, or in therange of 0.6 to 1.0 mm.

The inductance of the electrical machine, L_(machine), may be in therange of 15 to 100 μH, in the range of 20 to 90 μH, in the range of 30to 80 μH, or in the range of 40 to 70 μH.

The maximum rated temperature of the insulation, θ_(ins, max), may begreater than or equal to 370 K. θ_(ins, max) may be in the range of 400to 550 K, in the range of 410 to 540 K, in the range of 420 to 530 K, orin the range of 430 to 520 K.

The maximum frequency of the current received by the stator coils froman DC:AC converter during operation of the electrical machine, f_(max),may be greater than or equal to 1.0 kHz, greater than or equal to 1.1kHz, greater than or equal to 1.2 kHz, greater than or equal to 1.3 kHz,greater than or equal to 1.4 kHz, or greater than or equal to 1.5 kHz.f_(max) may be in the range of 1.0 to 2.0 kHz, in the range of 1.1 to1.9 kHz, in the range of 1.2 to 1.8 kHz, or in the range of 1.25 to 1.75kHz.

The angular speed of rotation of rotor of the electrical machine,ω_(mech,cont), when the electrical machine is producing the maximumcontinuous rated torque, τ_(max,cont), may be less than 200 rads⁻¹.τ_(max,cont) may be in the range of 75 to 150 rads⁻¹ (in the range of716 to 1432 rpm), in the range of 80 to 140 rads⁻¹ (in the range of 764to 1337 rpm), in the range of 90 to 130 rads⁻¹ (in the range of 859 to1241 rpm), or in the range of 100 to 120 rads⁻¹ (in the range of 955 to1146 rpm).

The angular speed of rotation of rotor of the electrical machine,ω_(hover), while the VTOL aircraft is hovering may be less than or equalto 160 rads⁻¹ (less than or equal to 1527 rpm). ω_(hover) may be in therange of 75 to 200 rads⁻¹ (in the range of 668 to 1910 rpm), in therange of 65 to 140 rads⁻¹ (in the range of 621 to 1337 rpm), in therange of 70 to 130 rads⁻¹ (in the range of 668 to 1241 rpm), in therange of 75 to 120 rads⁻¹ (in the range of 716 to 1146 rpm), or in therange of 80 to 110 rads⁻¹(in the range of 764 to 1050 rpm).

The maximum tip speed of the propeller or fan of the VTOL aircraft,ν_(tip), during take-off may be in the range of 130 to 250 ms⁻¹ (in therange of 0.38 to 0.73 Mach), in the range of 140 to 240 ms⁻¹ (in therange of 0.41 to 0.7 Mach), in the range of 150 to 230 ms⁻¹ (in therange of 0.43 to 0.67 Mach), in the range of 160 to 220 ms⁻¹ (in therange of 0.47 to 0.64 Mach), or in the range of 170 to 210 ms⁻¹ (in therange of 0.5 to 0.61 Mach).

The propeller or fan of an EPU may have a diameter of less than or equalto 5.0 meters, less than or equal to 4.5 meters, less than or equal to4.0 meters, less than or equal to 3.5 meters, less than or equal to 3.0meters, less than or equal to 2.5 meters, or less than or equal to 2.0meters. The diameter may be greater than or equal to 1 meter, greaterthan or equal to 1.5 meters, greater than or equal to 2.0 meters, orgreater than or equal to 2.5 meters.

The skilled person will appreciate that except where mutually exclusive,a feature described in relation to any one of the above aspects may beapplied mutatis mutandis to any other aspect. Further, except wheremutually exclusive, any feature described herein may be applied to anyaspect and/or combined with any other feature described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with referenceto the accompanying drawings, which are purely schematic and not toscale, and in which:

FIG. 1 is a perspective view of an example of a vertical take-off andlanding (VTOL) aircraft with six electrical propulsion units (EPUs);

FIG. 2 is a schematic illustration of an example of a propulsion systemthat may be used in the VTOL aircraft of FIG. 1 ;

FIG. 3 is schematic illustration of an example of a portion of thepropulsion system of FIG. 2 , showing the use of multiple power lanesper EPU;

FIG. 4 is a schematic illustration an example of an EPU, further showingthe use of multiple power lanes;

FIG. 5A is a schematic cross-section of an example of a radial fluxelectrical machine;

FIG. 5B shows flux paths of an example of magnetic circuits formed inthe radial flux electrical machine of FIG. 5A;

FIG. 6 is a plot illustrating an example of the optimization of thedesign of a radial flux electrical machine;

FIG. 7A is a schematic cross-section of an example of a transverse fluxelectrical machine;

FIG. 7B is a circumferential cross-section through the transverse fluxelectrical machine of FIG. 7A;

FIG. 8 illustrates the flux paths of the magnetic circuits formed in thetransverse flux electrical machine of FIGS. 7A-B;

FIG. 9A is a schematic illustration of an example of the active parts ofa stator of a transverse flux electrical machine that has three phasesand two coils per phase;

FIG. 9B is a more detailed view of an example of a single coil of thestator of FIG. 9A, further showing a portion of the stator supportstructure;

FIG. 10A is an example of a circuit diagram showing one way ofconnecting the stator coils of the electrical machine of FIG. 9A;

FIG. 10B is an example of a hybrid circuit illustration combining FIG.9A and FIG. 10A, showing the connection of the stator coils and the pathof current through the stator coils;

FIG. 11 is an example of a cross-section of an electrical machine havingthe stator of FIGS. 9 and 10 , further showing the rotor and additionalsupport structures;

FIG. 12A is a schematic illustration of an example of a transverse fluxelectrical machine with radial air gaps;

FIG. 12B is a perspective view of an example of one stator coil of thetransverse flux electrical machine of FIG. 12A;

FIG. 13 is a schematic illustration of an example of a transverse fluxelectrical machine in which a circumference of the stator is dividedinto sectors to implement first and second power lanes;

FIG. 14 is a cutaway view of an example of a transverse flux electricalmachine in which first and second power lanes are implemented usingaxial stacking of active parts;

FIG. 15 is an example of a plot illustrating how the tangential forcedeveloped in a rotary electrical machine varies with air gap size andpole pitch;

FIG. 16A is a perspective view of an example of a transverse fluxelectrical machine;

FIG. 16B is another perspective view of an example of a transverse fluxelectrical machine, further showing components of a clutch mechanism ofthe electrical machine;

FIG. 17 illustrates an example of a cross-section of the transverse fluxelectrical machine of FIGS. 16A-B, showing additional components and aspace in which the active parts of the machine are located;

FIG. 18A is perspective view of an example of a portion of the stator oftransverse flux electrical machine of FIG. 17 ;

FIG. 18B is a perspective view of an example of the portion of thestator of FIG. 18A, further showing the end windings;

FIG. 19A is a further illustration of an example of a portion of thestator of the electrical machine of FIG. 17 , showing how a stator slotmay be divided into discrete regions to aid cooling;

FIG. 19B is a schematic illustration of an example of how portions ofthe stator coil of FIGS. 18A-18B and 19A may be connected;

FIG. 20A is a top-down view of an example of one stator coil of theelectrical machine of FIGS. 16-19 ;

FIG. 20B is a side-on view of the stator coil of FIG. 20A;

FIG. 21 is a schematic cross-section of an example of an air-cooledmulti-lane transverse flux electrical machine;

FIG. 22 is a perspective cutaway of an example of the transverse fluxelectrical machine of FIGS. 16A-B and 17, further showing the activeparts;

FIG. 23A illustrates an example of how the conductor cross-section andslot packing of a transverse flux electrical machine may be varied tooptimize slot current density, torque generation, and cooling; and

FIG. 23B illustrates an additional example of how the conductorcross-section and slot packing of a transverse flux electrical machinemay be varied to optimize slot current density, torque generation, andcooling.

DETAILED DESCRIPTION

FIG. 1 illustrates a vertical take-off and landing (VTOL) aircraft 1that may be used for Urban Air Mobility (UAM) applications. The VTOLaircraft 1 includes a fuselage 20 that incorporates a cabin foroccupants, wings 30, a rear flight surface 40, and a distributedpropulsion system 10. The distributed propulsion system 10 includes sixelectrical propulsion units (EPUs), four of which are front EPUs 100 fand two of which are rear EPUs 100 r. Also visible in FIG. 1 is aretractable undercarriage 50 in which a landing platform or gear, inthis case having wheels, may be stowed during flight.

The size of the fuselage 20 and the cabin depends on the applicationrequirements. In this example, the cabin is sized for five occupantsincluding a pilot. Some UAM platforms, however, will not require a pilotand will instead be flown under the control of an autopilot system ormay be controlled remotely.

Each EPU 100 f, 100 r has a propeller 110 driven to rotate by anelectric motor. The four front EPUs 100 f are attached to the wings 30of the aircraft 1, and the two rear EPUs 100 r are attached to theseparate flight control surface 40 located towards the rear of theaircraft 1. The wings 30 and the rear control surface 40 are tiltablebetween a VTOL configuration (shown in FIG. 1 ) in which the axes of thepropellers of the EPUs point upward to provide vertical lift forvertical take-off and landing and a horizontal flight configuration inwhich the axes of the rotors point forward. The horizontal flightconfiguration, while principally used for horizontal flight, may also beused for taxiing and possibly short take-off and landing (STOL)operation if supported. In other examples, the wings 30 and/or the rearcontrol surface 40 may be fixed in a horizontal position, and the EPUsattached thereto may be tiltable in order to selectively switch betweena horizontal flight mode and a vertical flight mode.

The electrical systems, including the electric motors that drive theEPUs 110 f, 110 r of the aircraft 1, receive electrical power from oneor more battery packs and/or fuel cell packs located within the aircraft1. The battery packs and fuel cells packs may be located within anysuitable part or parts of the aircraft 1, including the EPUs 100 f, 100r, the fuselage 20, and the wings 30.

While the illustrated aircraft 1 is a VTOL aircraft, UAM platforms mayalso be of the STOL or conventional take-off and landing (CTOL) type.Further, while an electric VTOL (eVTOL) aircraft is shown, thepropulsion system may be a hybrid-electric propulsion system thatincludes both engines (e.g., one or more gas turbine engines) andbatteries and/or fuel cells. Hybrid-electric platforms may utilizesimilar distributed propulsion system configurations, but the underlyingpower system may be a series-hybrid, parallel-hybrid, turboelectric, orother type of hybrid power system.

The configuration of the illustrated VTOL aircraft 1 is merely oneexample configuration, and other VTOL aircraft configurations are knownand will occur to those skilled in the art. For example, a VTOL aircraftmay have a different number of EPUs (e.g., eight EPUs, with four frontEPUs 100 f and four rear EPUs 100 r). Alternatively, the VTOL aircraftmay have a multi-copter (e.g., quadcopter) configuration in which thepropellers or fans of the EPUs may not be tiltable and may be ducted.Other VTOL aircraft may have features of more than one type (e.g., a mixof open and ducted propulsors and/or a mix of tiltable and fixedpropulsors). The present disclosure is not limited to any particulartype of VTOL aircraft.

As noted above, each EPU of the six EPUs 100 f, 100 r includes apropeller or fan 110 driven to rotate by an electric motor that receiveselectrical power from an onboard power source. In some examples, eachmotor may receive the electrical power via its own dedicated powerchannel, possibly from its own dedicated power source (e.g., a batterymodule). In other examples, some of the EPUs may share a power channel.This is shown in FIG. 2 , which is a simplified illustration of anelectrical power and propulsion system 10 that may be used in the VTOLaircraft of FIG. 1 .

The power and propulsion system 10 includes three power and propulsionsub-systems 10 a, 10 b, 10 c. Each sub-system of the sub-systems 10 a-cincludes two of the six EPUs, and the two EPUs of each sub-system areelectrically connected to a shared power channel. The first sub-system10 a includes a first of the front EPUs 100 f-a and a first of the rearEPUs 100 r-a connected to first power channel 140 a. The secondsub-system 10 b includes a second of the front EPUs 100 f-b and a secondof the rear EPUs 100 r-b connected to a second power channel 140 b. Thethird sub-system 10 c includes the remaining two front EPUs 100 f-c, 100f-d, (e.g., one front-left EPU and one front-right EPU), connected to athird power channel 140 c. Each power channel 140 c receives DCelectrical power from a battery module 150 a-c. The battery modules 150a-c may be physically separate from one another or may be part of acommon battery pack that outputs three separate power channels 140 a-c.

In the present example, the power channels 140 a-c receive the DCelectrical power directly from the battery modules 150 a-c. In otherexamples, the battery modules 150 a-c may interface with the powerchannels 140 a-c via DC:DC power electronics converters. This may allowthe DC voltage level of the power channels to be kept substantiallyconstant as the state of charge of the battery modules 150 a-cdecreases. Also, while in the present example the sole power source isin the form of battery energy storage 150 a-c, alternative examples mayinclude only fuel cells, or a mix of fuel cells and battery energystorage. In a hybrid system, the power source may include one or moreengine-driven electric generators interfacing with the DC power channelsvia DC:AC power electronics converters (e.g., rectifiers).

Each EPU 100 f-a, 100 f-b, 100 f-c, 100 f-d, 100 r-a, 100 r-b includes apropeller or fan 110 with rotation that is driven by an electric motor120. The motor 120 receives AC electrical power from a DC:AC powerelectronics converter 130 (e.g., an inverter 130). The inverter 130receives DC electrical power from its power channel 140 a-c and convertsthe DC electrical power to AC electrical power for supply to its motor120.

Aircraft and power and propulsion systems of the aircraft are subject tostrict safety and certification requirements. An aircraft and itssafety-critical systems may be tolerant to faults; the aircraft and itssafety-critical systems may be capable of continued, safe operationafter the failure of a component. To this end, FIG. 3 illustrates theprinciple of a laned architecture that may be adopted for the power andpropulsion system 10 of FIGS. 1 and 2 to improve fault tolerance.

FIG. 3 shows a power and propulsion sub-system 10 n that has a lanedarchitecture. The sub-system 10 n may be any of the sub-systems 10 a-cof FIG. 2 . As in FIG. 2 , the sub-system 10 n of FIG. 3 includes twoEPUs 100, each of which includes a propeller or fan 110 driven to rotateby an electric motor 120. However, unlike the motors of FIG. 2 , themotors 120 of FIG. 3 are multi-lane electric motors. In particular, themotors 120 of FIG. 3 are dual-lane motors with two electricallyindependent AC inputs (e.g., two independent three-phase inputs).

The term “multi-lane electric motor” or “multi-lane electrical machine,”as used herein, refers to an electric motor that has at least twoelectrically independent sets of stator coils that may be separatelyexcited and may separately interact with a rotor to produce torque. Inthis way, in the event of a fault in one lane of the motor (e.g., astator terminal short circuit), the remaining lane(s) may remainfunctional, and the multi-lane motor 120 may thus continue to applytorque to rotate the propeller or fan 110. Depending on the number oflanes and the extent to which the motor 120 is overrated, the motor 120may be able to supply all (i.e., 100%) or a large proportion (e.g., 70%,80%, or 90%) of the torque that would be supplied to rotate thepropeller or fan 110 during normal, fault-free operation. In thisexample, the motors 120 have two lanes and may be referred to asdual-lane motors, but a higher number of lanes (e.g., three or fourlanes) may be used. The combination of an independent set of statorcoils and an associated rotor with which the stator coils interact maybe referred to as one lane of the multi-lane electric motor, or as a“sub-machine” of the multi-lane electrical machine.

A multi-lane motor may take one of a number of different forms. In oneexample, the dual-lane motor of FIG. 3 takes the form of two axiallystacked motors having rotors that are mechanically coupled to the sameoutput shaft. In this case, the motor has two completely separate setsof active parts (e.g., two axially spaced stators and two axially spacedrotors) but has some shared structures and features, (e.g., sharedsupport structures, casing, and cooling features). In another example,each dual-lane motor of FIG. 3 has a single stator structure having acircumference that houses two electrically independent sets of coils,one set belonging to the first sub-machine and the second set belongingto the second sub-machine. The first set of coils and the second set ofcoils may interact with a common rotor. The above described approachesmay be extended to a higher number of lanes (e.g., three axially stackedsub-machines) or may be combined to give a higher number of lanes (e.g.,four lanes formed from two axially stacked arrangements, each having twosub-machines). Other examples will occur to those skilled in the art.

FIG. 3 also shows that each lane of the dual-lane electric motor 120receives its multi-phase AC input from a separate, independent DC:ACinverter 130 i, 130 ii. Likewise, each inverter 130 i, 130 ii receivesits DC input from a separate, independent DC power channel 140 i, 140 iithat itself receives power from its own battery module 150 i, 150 ii.Thus, the propulsion sub-system 10 n is not only tolerant to faults inthe multi-lane electric motor 120, but is also tolerant to faults inother sub-system components. For example, a failure in a convertercircuit 130 i of the first lane will not prevent the supply ofelectrical power to the second lane of the associated motor 120.Likewise, the loss of one battery module 150 i or a fault in a powerchannel 140 i will not prevent the operation of the inverters 130 ii ofthe second lane or the second lanes of the motors 120. Thus, the use ofa laned architecture, including multi-lane motors 120, increases theredundancy in and fault tolerance of the power and propulsion system 10.

Other measures, not shown in the simplified systems 10, 10 n of FIGS.2-3 , are also possible. For example, a propulsion system 10 may providefor a degree of reconfigurability to improve the fault tolerance andpower availability within the system. For example, in the event of afailure of one battery module (e.g., battery module 150 i), thesub-system 10 n may be reconfigured (e.g., by the selective opening andclosing of switches such as contactors or solid-state power controllers(SSPCs)) to allow electrical power from the second battery module 150 iito be supplied to the first power channel 140 i. As another example,following a fault in the first lane of one or both of motors 120, thesub-system 10 n may be reconfigured to allow electrical power from thefirst battery module 150 ii to be supplied to the second power channel140 ii.

FIG. 4 is a schematic illustration of a multi-lane EPU 100 such as maybe used in the VTOL aircraft and propulsion systems of FIGS. 1-3 . TheEPU 100 includes a propeller or fan 110 mechanically coupled to, andthereby drivable by, a rotor of a multi-lane electric motor 120. In thisexample, the multi-lane motor 120 is a dual-lane motor in which each ofthe two sub-machines has three phases. Thus, the motor 120 is shown ashaving two three-phase inputs that receive power from the outputs of twoDC:AC converters 130 i, 130 ii. The DC:AC converters 130 i, 130 iiinterface with respective DC power channels 140 i, 140 ii, the positiveand negative rails of which are indicated.

The inverters 130 i, 130 ii are controlled by controllers 135 i, 135 ii.The controllers 135 i, 135 ii may, for example, control the switchingfrequencies, switching timings, and duty cycles of MOSFETs of theinverters 130 i, 130 ii to adjust the magnitudes and frequency of theoutput AC voltage and current waveforms of the inverters 130 i, 130 ii.In this way, the controllers 135 i, 135 ii may control the lanes of themotor 120 to produce the required torque, for example. In this example,each lane has its own controller; again, this prevents a fault in onecontroller (e.g., controller 135 i) from impacting the entire EPU 100.

FIG. 4 also shows an EPU cooling system 125. The EPU cooling system 125is responsible for managing the temperature of the motor 120 and theinverters 130 i, 130 ii during use. For example, the cooling system 125provides that the temperature of the insulation of the stator coils doesnot exceed its rated temperature. The cooling system 125 may take anysuitable form, including a liquid cooling system that utilizes a pumpedliquid coolant (e.g., an oil) or an air cooling system that directs aflow of air (e.g., ambient air) to cool the components. In someexamples, the cooling system 125 includes substantially separate coolingsystems for cooling the electrical machine 120 and the inverters 130 i,130 ii. In other examples, the cooling system 125 is shared by theelectrical machine 120 and the inverters 130 i, 130 ii. Further, in someexamples, the cooling system 125 may include separate parts for eachlane of the EPU 100 to prevent a cooling system fault from impacting theentire EPU 100.

The EPU 110 may include a gearbox 105. The optional gearbox 105 may berequired to step down the speed of the rotor(s) of the motor 120 to alower speed of rotation for the propeller or fan 110. VTOL aircraft areexpected to have relatively large propellers or fans 110 (e.g.,diameters of 2-4 meters) in order to limit disk loading while increasingpropulsive efficiency during VTOL and hover. At the same time, there isa desire to keep aerodynamic noise low, wherein the aerodynamic noise isstrongly dependent on the propeller tip speed. The combination of alarge propeller diameter and a low propeller tip speed necessitates arelatively low propeller rotational speed. Unless the electrical machine120 is capable of providing the required torque at a low rotationalspeed, which is relatively high given the large propeller, a gearbox 105is to be provided.

Also shown in FIG. 4 are a propeller bearing unit 111 and an EPUlubrication system 115. The presence of and designs of the propellerbearing unit 111 and the EPU lubrication system 115 will depend on thedesign of the EPU 110 and are beyond the scope of the present disclosureand will not described any further.

The VTOL aircraft 1, the propulsion system 10, and the EPU 100 describedwith reference to FIGS. 1-4 are only intended to be examples, and manyother arrangements are possible and within the scope of the presentdisclosure. As noted previously, the aircraft 1 may additionally oralternatively include fuel cells or engines. The aircraft 1 may also beof a different VTOL design (e.g., multi-copter or different number andarrangement of EPUs) or utilize a different electrical power systemlayout. However, the above description explains certain principles of aVTOL aircraft.

The general design of VTOL aircraft, such as the one described above,results in a number of design challenges. Some of these are discussedbelow.

In one example, a need for redundancy and propulsive efficiency callsfor a distributed propulsion system with a relatively large number ofEPUs. In the above example, there are six EPUs, and most proposed VTOLaircraft designs include at least four EPUs. This increases the mass andreduces the power density of the VTOL aircraft because each EPU includesnot only lift- and thrust- producing parts but support and attachmentstructures, cabling, etc.

In a second example, high torque and low speed requirements of thepropeller or fan of the EPU, discussed above, may call for a gearbox tostep down the output rotor speed of the electrical machine. A gearbox isa relatively heavy component and also introduces additional complexityas well as lubrication and maintenance requirements. Further, each EPUwould require its own gearbox, multiplying the gearbox mass by, forexample, six times. A direct drive arrangement would eliminate this massand complexity and be of great advantage. However, designing a hightorque, low speed electrical machine that is lightweight and efficient,yet does not have onerous cooling requirements, is a challenge.

Table 1 provides exemplary values of a hover parameter, Ψ, that may beachieved by motors described herein. The hover parameter Ψ is defined(see Equation (26)) as the continuous torque produced by the motor whilethe VTOL aircraft is hovering (τ_(hover)) divided by the continuousangular speed of rotation of the rotor of the motor while the VTOLaircraft is hovering (ω_(hover)), measured in radians per second. By“hovering,” it is provided that the EPUs of the VTOL aircraft areproducing sufficient thrust to lift the aircraft and maintain a constantaircraft altitude, without requiring the assistance of airframe lift(e.g., wing-borne lift).

TABLE 1 Ψ(Nmsrad⁻¹) Example 1 Example 2 Example 3 7.2 13.1 16.3

Motors described herein may have values of Ψ in the range 5 to 20Nmsrad⁻¹, which may allow for omission of the gearbox 105, resulting inreduced EPU and aircraft mass.

In another example, strict safety and certification requirements ofaircraft call for a fault-tolerant electrical power system (e.g., thelaned architecture described above with reference to FIGS. 3 and 4 ).This results in further multiplication (e.g., duplication) of componentsin an EPU: electrical machines with multiple sets of active parts;multiple power channels; multiple inverters; and multiple coolingsystems. This results in a further increase in EPU mass. It would bedesirable to reduce the mass of the active parts and cooling systemassociated with the electrical machine, to the extent that this ispossible, while meeting the platform design requirements.

Table 2 provides examples of values of a take-off parameter, x, that maybe achieved by a VTOL aircraft with one or more EPUs incorporating anelectric motor described herein. The take-off parameter, x, is defined(see Equation (24)) as the maximum tip speed (ν_(tip)), measured inms⁻¹, of the propeller or fan of the EPU to occur during a verticaltake-off operation of the VTOL aircraft divided by the active partstorque density (p_(act)—see Equation (1)).

TABLE 2 χ (sm⁻¹) Example 1 Example 2 Example 3 1.2 2.1 4.7

Motors described herein may have take-off parameters in the range of 0.5to 7.5 sm⁻¹, which may be associated with reduced system mass and noise.

In another example, multiplication of electrical components such asinverters results in a stacking of losses in the system. For example, apropulsion system with six EPUs and a dual lane architecture wouldinclude at least twelve inverters, each having its own losses. Onemitigation is to design inverters with high efficiencies, for example,by using state-of-the-art semiconductor materials. Even then, however,it would be desirable to be able to operate the inverters in anoperating regime close to their peak efficiency, which occurs when theelectrical frequency of the inverter output waveform is relatively high.This is a design challenge, especially for direct drive, because itrequires the use of a high inverter output frequency with a low rotorspeed of rotation.

Electrical machine designs that are optimized for aircraft propulsionsystems, particularly for VTOL aircraft, and may address one or more ofthe above problems and/or other problems are now described withreference to FIGS. 5A-23 .

FIG. 5A illustrates a radial flux permanent magnet motor 200 (e.g., amotor) that may be used for EPUs of electric aircraft, including VTOLaircraft. As noted previously, permanent magnet motors may be providedfor VTOL applications because their power density is relatively highcompared with most other designs.

For clarity, FIG. 5A only shows the active parts of the motor 200.“Active parts” refers to the components of the motor 200 that contributeto the production of torque. Thus, “active parts” include magnetic fluxgenerating components such as coils and magnets, and magnetic fluxguiding components such iron, but “active parts” do not include supportstructures, cooling system features, etc., which do not contribute tothe production of torque. The active parts of the motor 200 have acumulated mass of m_(act), referred to herein as the active parts mass.For the avoidance of doubt, the active parts mass includes the mass ofthe end windings and coil insulation because these are integral featuresof the coils without which a motor cannot produce any torque.

The radial flux motor 200 includes a stator 210 and a rotor 220 arrangedto rotate about an axis of rotation 230.

The stator 210 includes an annular stator back iron 211, which may alsobe referred to as a yoke, and a plurality of circumferentially arrangedstator teeth 212 (e.g., stator teeth 212) that project radially inwardlyfrom the back iron 211. The stator teeth 212 define stator slots 213,which may also be referred to as stator winding space, between thestator teeth 212. In the present example, there are twenty-four statorteeth 212 defining twenty-four stator slots 213 therebetween. The statorteeth 212 and/or the stator iron 211 may, for example, be formed oflaminations of a ferromagnetic material to improve their flux guidingperformance while reducing the induction of eddy currents in the statorteeth 212 and/or the stator iron 211. In another example, the fluxguiding material includes a soft magnetic composite (SMC) such as, forexample, a non-iron material with embedded iron particles. The activeparts mass, m_(act), of the motor 200 includes any carrier material ofthe flux guiding iron (e.g., non-iron material included in laminations)or the non-iron material in an SMC.

The stator 210 further includes a plurality of electrically conductivestator coils 214 (e.g., stator coils) wound around the stator teeth 212.The stator coils 214 may be formed of any suitable material, such ascopper or aluminum. Strands of the conductor that form the stator coils214 have (e.g., are coated in) an electrically insulating material toprevent short circuits. In the present example, there are twelve statorcoils 214, and each coil occupies two of the slots 213.

In this example, the motor 200 is a three-phase motor, and thus thereare 12/3=4 stator coils 214 per phase. The three phases are designatedU, V, Win FIG. 5A, and the stator coils 214 are labelled 1-4. Statorcoils 214 of the same phase (e.g., U1, U2, U3, U4) are evenlydistributed about the circumference of the stator 210, whilecircumferentially adjacent stator coils 214 (e.g., U1 and V1, or V1 andW1) belong to different phases. Many different stator windingarrangements are known, but, in this example, a distributed windingarrangement in which each coil is wound around two teeth that arelocated 2Π/8=Π/4 radians (45 degrees) apart is used. Stator coils 214 ofthe same phase are electrically connected (e.g., in series or inparallel). The terminals of each set of phase coils may be connected in,for example, a star or delta configuration, and the input terminals maybe connected to an inverter (e.g., a two-level, three phase bridgecircuit) from which the stator coils 214 receive current. In analternative example, for increased fault tolerance, the stator coils 214of each phase may be connected to its own inverter circuit (e.g., anH-bridge circuit).

The rotor 220 includes an annular rotor back iron 221 and a plurality ofpermanent magnets 222 (e.g., permanent magnets) arranged around acircumference of the rotor 220 forming a plurality of permanent magnetrotor poles (e.g., permanent magnet poles). Circumferentially adjacentpermanent magnet poles are of opposite polarity. The permanent magnetpoles are distributed evenly about the rotor circumference and define apole pitch, P_(θ), equal to 2Π divided by the number of permanent magnetpoles (N_(p)):

$\begin{matrix}{P_{\theta} = \frac{2\pi}{N_{P}}} & (27)\end{matrix}$

In this example, there are eight permanent magnet poles, so P_(θ) isequal to 2Π/8=Π/4 radians (45 degrees).

In addition to the pole pitch, P_(θ), a pole arc length, P_(L), of themotor is also defined. Herein, the pole arc length is equal to polepitch, P_(θ), measured in radians, multiplied by the active partsradius, the active parts radius being half the active parts diameter,D_(Act), of the motor:

$\begin{matrix}{P_{L} = \frac{P_{\theta} \times D_{Act}}{2}} & (28)\end{matrix}$

The active parts diameter, D_(Act), is a diameter corresponding to aradially outermost active part of the motor 200. In this example, inwhich the rotor 220 is radially inward of the stator 210, a radiallyouter circumference of the stator iron 211 defines the active partsdiameter. When the rotor 220 is instead radially outward of the stator210, a radially outermost active part of the rotor defines the activeparts diameter. In the present example, if the motor 200 is sized for anEPU of a VTOL aircraft, the active parts diameter may be about 0.45meters, giving a pole arc length, P_(L), of about 17.7 cm.

FIG. 5A also labels an air gap 215 that separates the rotor 220 from thestator 210. The air gap 215 has a width, measured in the radialdirection for the radial flux motor 200, equal to G_(Air).

In use, the stator coils 214 of the stator 210 are excited with AC powerto generate a rotating magnetic field that interacts with the magneticfield of the permanent magnets 222 to produce torque. The torque causesthe rotor 220 to rotate relative to the stator 210 about the axis ofrotation 230. The motor 200 is configured to produce a maximumcontinuous rated torque of τ_(max,cont) and a peak rated torque ofτ_(peak). As used herein, τ_(max,cont) is the highest torque the motorcan produce and sustain for an extended period (e.g., at least threeminutes) at ISA sea level conditions. τ_(max,cont) depends to someextent on the capabilities of the cooling system of the motor, which isconfigured to remove heat at a rate sufficient to maintain thetemperature of the stator coil insulation below its rated temperaturewhile operating at τ_(max,cont) As used herein, τ_(peak) is a highesttorque the motor can produce for a short period (e.g., a three secondtransient period) at ISA sea level conditions without damaging the motordue to, for example, breakdown of the coil insulation due to excessivevoltage or excessive heat generation.

FIG. 5B illustrates main magnetic circuits produced by the radial fluxmotor 200 during use. The current flowing through each stator coil 214produces magnetic flux that flows in a radial direction. For a givenstator slot 213, the flux produced by the stator coil 214 flows radiallyoutward along one tooth 212 and radially inward along another tooth 212on a circumferentially opposite side of the slot 213. At the radiallyouter side of the stator 210, the magnetic flux flows circumferentiallyalong the stator back iron 211 between stator teeth 212. At the radiallyinner side of the stator 210, the magnetic flux crosses the air gap 215to flow to/from a stator tooth 212 from/to a permanent magnet 222 of therotor 220. In the rotor 210, flux flows from a permanent magnet 222 tothe rotor iron 221 and then flows circumferentially along the rotor iron221 to another permanent magnet 222.

FIG. 6 is a plot 1000 illustrating how the design of a radial flux motor200 may be optimized for use in a EPU of VTOL aircraft. Athree-dimensional surface 1000 is shown, with shading of the surface1000 indicating the temperature of the insulation of the stator coils214.

The vertical axis represents the active parts torque density, p_(act),of the motor, defined in Equation (1) as the peak rated torque dividedby the cumulated mass of the active parts (i.e., the components whichcontribute to the production of torque) measured in Nmkg⁻¹. In the caseof VTOL aircraft, it desirable for the active parts torque density,p_(act), to be high because this implies the platform's torqueproduction requirements are met at a low motor mass, which is asignificant benefit in VTOL aircraft due in part to the multiplicationof components (e.g., multiple EPUs). The remaining two axes show theslot current density, J_(slot,peak), (e.g., the slot current densitywhen producing the peak rated torque), measured in A(mm)⁻², and thelinear RMS current loading, A_(rms), measured in kA/m. In certainexamples, the higher the current loading and the slot current density,the higher the torque production. However, if the current density ishigh, the stator coil temperature will be higher for a given nominalcooling rate because resistive losses (i.e., |²R losses) will also behigher.

On the surface 1000, an isotherm 1001 (i.e., a line of constanttemperature) is shown. The isotherm 1001 represents operation at therated temperature of the insulation, assuming operation of a liquidcooling system that cools the stator coils at a nominal rate. In otherwords, the isotherm 1001 divides the surface 1000 into two design spaceregions: a lower region 1002 below the isotherm 1001 in which operationis sustainable at the nominal cooling rate; and an upper region 1003above the isotherm 1001 in which operation is not sustainable at thenominal cooling rate. Thus, the isotherm 1001 may be regarded as theoptimal design. FIG. 6 further shows three possible stator tooth andslot designs, labelled (i), (ii) and (iii), and operating points thereofthat lie on the isotherm 1001.

First referring to design (i), this shows a stator tooth 212 that isrelatively long in the radial direction and includes a large volume ofconductor in the slots 213 defined circumferentially adjacent to thetooth 212. The large volume of active parts (e.g., the iron stator teethand the conductor) results in high torque for a given slot currentdensity. However, the large volume of active parts also results in ahigh active parts mass, which limits the active parts torque densityp_(act). Further, the use of radially long stator teeth 212 providesthat, for a motor of a given diameter (noting the diameter will beconstrained by the EPU integration requirement), there is relativelylittle space to flow coolant around the active parts. Thus, while theslot current density is low for a given current value, the extent towhich the slot current density may be increased without departing fromthe isotherm 1001 into the region 1003 is limited.

Design (ii) is a more optimized design for VTOL aircraft in that design(ii) better balances torque production, slot current density, and activeparts mass. As shown, compared with design (i), design (ii) has radiallyshorter teeth 212 with a smaller volume of conductor in the slots. Whilethis reduces torque production at a given value of the slot currentdensity, radially shorter teeth 212 with a smaller volume of conductorin the slots reduces the active parts mass. This also providesadditional room for coolant, which improves cooling and therefore allowsfor an increase in the slot current density without departing fromisotherm 1001 into the upper region 1003. Further, the radially shorterteeth have a lower aspect ratio, which may improve flux guiding, andallow for the use of a larger radius rotor. The use of a larger radiusrotor may produce a higher torque. Overall, as shown, tooth design (ii)has the peak value of p_(act) on the isotherm 1001.

Design (iii) illustrates the impact of further reducing the radiallength of the stator tooth 212 and decreasing the volume of conductor inthe slot. As before, the reduction in active parts volume results inlower torque production at a given slot current density but also areduction in active parts mass. The additional free volume for coolantallows the slot current density to be increased, thus increasing activeparts torque density p_(act), without departing from the isotherm 1001.However, resistive losses increase with the square of the currentdensity whereas the torque increases in a linear fashion. There istherefore a point on the isotherm 1001 after which the increase intorque that results from the increase in slot current density, and whichis made possible by the increase in cooling volume, does not compensatethe reduction in torque that results from the reduced volume ofconductor. Therefore, design (iii) is associated with a lower value ofp_(act) than design (ii).

While optimized radial flux designs may be used in the EPUs of VTOLaircraft, further performance improvements and optimizations may beprovided. To this end, FIG. 7A, illustrates a transverse flux electricmotor 300 that may be particularly suitable for use in VTOL aircraft.FIG. 7B is a circumferential cross-section of the motor 300 of FIG. 7A.As before, for clarity and ease of explanation, only the active parts ofthe motor 300 are shown. The illustrated motor 300 has only a singlephase. This is for clarity of explanation; in practice, a motor may havea higher number of phases, and such a motor will be described below.

The transverse flux motor 300 has a stator 310 and a rotor 320 a, 320 barranged to rotate about an axis of rotation 330.

The stator 310 includes flux guiding stator iron 311 that defines acircumferentially extending stator slot 313 (e.g., generally annularstator slot; annular winding space). In this example, the stator iron311 includes a plurality of circumferentially arranged flux guidingstator elements 312 (e.g., stator elements 312) that together define andsurround the annular winding space 313. In the present example, thereare eight stator elements 312, alternately facing axially up and axiallydown, defining a single stator slot 313. The stator elements 312 may beformed of laminations of a ferromagnetic material or an SMC to improvetheir flux guiding performance while reducing the induction of eddycurrents. In other examples, the winding space 313 may be defined by acontinuous (e.g., a single piece) stator iron structure as is the casein the radial flux machine of FIG. 5A.

The stator slot 313 houses a circumferentially extending stator coil314. As in the radial flux motor 200, the stator coil 314 may be formedof any suitable material such as copper or aluminum. The conductor thatforms the stator coil 314 has (e.g., is coated in) an electricallyinsulating material to prevent short circuits. In FIGS. 7A and 7B, thestator coil 314 is a solid piece of conductor (e.g., the coil has asingle turn). In practice, the stator coil 314 may have a plurality ofturns; this is discussed in more detail below.

FIG. 7B is a circumferential cross-section through the active parts ofthe transverse flux motor 300, and more clearly shows how the fluxguiding stator elements 312 may define the open slot 313. In FIG. 7B,two circumferentially adjacent stator elements 312 i, 312 ii defining asingle stator pole are shown. Stator element 312 i is in the foreground,and stator element 312 ii is in the background. Each of the statorelements 312 i, 312 ii has a generally claw-shaped form factor andincludes a body portion 3121 and two projections 3122, 3123 that projectfrom the body portion 3121. In this example, the projections 3122, 3123extend axially away from the body portion 3121, and the projections3122, 3123 of circumferentially adjacent stator elements 312 i, 312 iiface axially opposite directions. In FIG. 7B, the projections 3122, 3123of the first stator element 312 i project axially downwards, whereas theprojections 3122, 3123 of the second stator element 312 ii projectaxially upwards. In this way, the two circumferentially adjacent statorelements 312 i, 312 ii cooperate to define the cross-section of awinding space (e.g., a slot 313) therebetween, which in this case has ahexagonal shape, though other shapes may be formed by modifying theshape and curvature of the stator elements 312 i, 312 ii. Collectively,the eight stator elements 312 of the stator 310 (see FIG. 7A) define anannular winding space, and the stator coil 314 is housed in the annularwinding space. Other stator element form factors are possible and inaccordance with the present disclosure.

The rotor 320 a, 320 b, is a dual rotor and has two rotor portions: aninner rotor portion 320 a that is radially inside the stator 310; and anouter rotor portion 320 b that is radially outside the stator 310. Inthis example, the inner rotor portion 320 a and the outer rotor portion320 b are mechanically connected so that the inner rotor portion 320 andthe outer rotor portion 320 b rotate together. Each of the inner rotorportion 320 a and the outer rotor portion 320 b includes a plurality ofcircumferentially arranged permanent magnets 322 a, 322 b definingevenly spaced permanent magnet poles (e.g., poles). Circumferentiallyadjacent poles are of opposite polarity. In this example, the innerrotor portion 320 a includes eight permanent magnet poles, and the outerrotor portion 320 b includes eight permanent magnet poles. Thus, in thepresent example, the pole pitch, P_(θ), of the motor 300 is 2Π/8=Π/4radians (45 degrees).

The permanent magnets 322 a of the inner rotor portion 320 a are affixedto an outer surface of an inner rotor structure 321 a. Similarly, thepermanent magnets 322 b of the inner rotor portion 320 b are affixed toan inner surface of an outer rotor structure 321 b. The inner rotorstructure 321 a and the outer rotor structure 321 b may include fluxguiding stator iron (e.g., laminations of a ferromagnetic material).However, the use of a dual rotor design, with permanent magnets 322 a,322 b on both radial sides of the stator 310, may allow for the omissionof iron material from the rotor 320 a, 320 b because the permanentmagnets 322 a, 322 b may define closed magnetic circuits. This isdescribed in more detail below. Other transverse flux motors 300 inaccordance with the present disclosure may not feature a dual rotor,and, in this case, rotor iron or additional stator iron may be providedto define closed magnetic circuits.

As stated above, the pole arc length, P_(L), of the motor is defined asthe pole pitch, P_(θ), measured in radians, multiplied by half theactive parts diameter, D_(Act), of the motor. In this example, whichfeatures an ironless dual rotor 320, the active parts diameter isdefined by the outer diameter of the permanent magnets 322 b of theouter rotor portion. Assuming the motor 300 is sized for an EPU of aVTOL aircraft, the active parts diameter may be about 0.45 meters,giving a pole arc length, P_(L), of about 17.7 cm.

FIGS. 7A-7B also label air gaps 315 a, 315 b that separate the innerrotor portion 320 a and the outer rotor portion 320 b from the stator310. The inner air gap 315 a has a width, for example, measured in theradial direction, equal to G_(Air,1). The outer air gap 315 b has awidth, for example, measured in the radial direction, equal toG_(Air,2). The air gap widths G_(Air,1), G_(Air,2) may be the same toequalize the motor loading.

The transverse flux motor 300 of the present example has radial air gaps315 a, 315 b. In other words, the two rotor portions 320 a, 320 b are onradially opposite sides of the stator 310. In other examples, atransverse flux motor has axial air gaps. In other words, the two rotorportions would be on axially opposite sides of the stator. Such anexample will be described with reference to FIGS. 9-11 .

The transverse flux motor 300 of the present example has only a smallnumber of pole pairs and relatively large values for the pole pitch,P_(θ), and pole arc length, P_(L). This is for ease of explanation. Aswill be described in detail below, the present disclosure provides theselection of a larger number of pole pairs to improve thecharacteristics of the motor.

FIG. 8 shows the magnetic flux paths of the main magnetic circuitsformed in the transverse flux motor 300 of FIGS. 7A-B. The left-handdiagram of FIG. 8 is the axial end view of FIG. 7A, with the mainmagnetic circuits overlaid and labelled by the magnetic flux densityvector {right arrow over (B)}. The dotted lines indicate where the fluxlines are below the plane of the page. The right-hand diagram, takenthrough plane A-A, is a zoomed-in perspective view of twocircumferentially adjacent stator elements 312, forming a single statorpole, and illustrates the three-dimensional shape of the flux path{right arrow over (B)}. The current and force vectors {right arrow over(I )} and {right arrow over (F)} are also shown.

The current flows through the stator coils 314 in the circumferentialdirection. This is illustrated in FIG. 8 by the current vector {rightarrow over (I)}. At each circumferential position, the current flowproduces a loop of magnetic flux in a plane perpendicular to thedirection of current flow. Considering a circumferential positioncorresponding to a first stator element 312 i, the loop of flux isguided along the radially extending body portion 3121 i of the statorelement 312 i before entering an axial projection of the stator element312 i. From there, the flux crosses the inner air gap 315 a and enters apermanent magnet 322 a of the inner rotor portion 320 a. The flux thenflows circumferentially through the inner rotor magnets 322 a to reach acircumferential position corresponding to the adjacent second statorelement 312 ii. From there, the flux again crosses the inner air gap 315a, this time into the second stator element 312 ii. The flux is thenguided along the radially extending body portion 3121 ii of the secondstator element 312 ii before entering an axial projection of the secondstator element 312 ii. From there, the flux crosses the outer air gap315 b and enters a permanent magnet 322 b of the outer rotor portion 320b. The flux then flows circumferentially through the outer rotor magnets322 b to reach another circumferential position (e.g., in this case,back to the circumferential position corresponding to the first statorelement 312 i). From here, the flux again crosses the outer air gap 315b to the first stator element 312 i, thus completing the magneticcircuit. The left-hand drawing of FIG. 8 shows a similar magneticcircuit for each adjacent pair of stator elements 312.

Thus, FIG. 8 shows that the magnetic circuits of the transverse fluxmotor 300 are three-dimensional (e.g., the magnetic circuits havecomponents in the radial, axial, and circumferential directions) andspiral around the annular stator winding region 313 that houses thestator coil 314.

As mentioned previously, a practical motor will include more phases thanthe single phase shown in FIG. 7A. As also mentioned previously, atransverse flux electrical machine may alternatively utilize axial airgaps instead of radial air gaps. To this end, a three-phase axial airgap transverse flux motor 60 will be described with reference to FIGS.9-11 .

FIG. 9A in axial end view of the active parts of a three-phase stator 61of a transverse flux motor 60 that has axial air gaps.

In this example, the three-phase stator 61 includes sixcircumferentially arranged phase modules 610-1 to 610-6, distributedevenly about the stator circumference. Radially opposite phase modules(e.g., phase modules 610-1 and 610-4) are associated with the same phase(e.g., phase U) of the motor 60 to provide mechanical balance. Eachphase module 610-1 to 610-6 includes flux guiding stator iron 611defining a circumferentially extending and open winding space 613 (e.g.,a slot), and a coil 614 (e.g., a stator coil) housed within the slot613.

FIG. 9B shows one of the phase modules 610 fixed to a support structure640. In this example, the slot 613 and the coil 614 housed within theslot 613 include first and second radially spaced portions.Specifically, the coil 614 includes a first, radially inner andcircumferentially extending, coil portion 614 a housed within a first,radially inner and circumferentially extending, slot portion 613 a. Thecoil 614 further includes a second, radially outer and circumferentiallyextending, coil portion 614 b housed within a second, radially outer andcircumferentially extending, slot portion 613 b. The first coil portion614 a and the second coil portion 614 b are connected at respectivecircumferential ends by end windings 617. In this way, the currentflowing through a coil 614 changes direction in the end windings 617,and the current flows through the first coil portion 614 a in acircumferential direction opposite to (e.g., generally antiparallel to)the current that flows through the second coil portion 614 b.

The flux guiding stator iron 611 includes two sets of flux guidingstator elements: a radially inner first set of flux guiding statorelements 612 a and a radially outer second set of flux guiding statorelements 612 b. The radially inner first set of stator elements 612 adefine the radially outer first slot portion 613 a that houses the firstcoil portion 614 a. The radially outer second set of stator elements 612b defines the radially outer second slot portion 613 b that houses thesecond coil portion 614 b. Each of the two sets of stator elements 612a, 612 b includes a plurality of circumferentially arranged, evenlydistributed stator elements 612. In the present example of FIG. 9B, thefirst, radially inner set of stator elements 612 a has twenty-fivestator elements 612, whereas the second, radially outer set of statorelements 612 b has twenty-seven stator elements 612. Other numbers ofstator elements are possible.

Each stator element 612 is substantially as described above withreference to FIGS. 7-8 (e.g., each element 612 includes a body portion3121 and two projections 3122, 3123). However, in the example of FIGS.9-11 , the stator elements 612 are oriented so that the body portions3121 extend axially and the projections 3122, 3123 extend radially. Theprojections 3122, 3123 of circumferentially adjacent stator elements 612i, 612 ii of each set 612 a, 612 b alternately project radially inwardlyand radially outwardly so as to define an open slot cross-section,similar to that shown in FIG. 7B but with the radial direction and theaxial direction swapped. Collectively, all of the stator elements 612within a set (e.g., set 612 a) define a slot portion (e.g., radiallyinner slot portion 613 a).

FIG. 10A is a circuit diagram of the stator 61 illustrating how thecoils 614 of the six phase modules 610-1 to 610-6 may be connectedtogether.

The stator 61 has three phase terminals U, V, W by which the stator 61receives AC electrical power from an inverter arrangement. For example,each phase terminal may be connected to a two-level, one-phase H-bridgeinverter circuit, or each phase terminal may be connected to one of thephase legs of a two-level, three-phase DC:AC inverter circuit.

The coil 614 of each phase module 610-1 to 610-6 has two terminals.Respective first terminals of the coils 614 of the radially oppositefirst phase module 610-1 and fourth phase module 610-4 are connected inparallel to the first phase terminal U. Respective first terminals ofthe coils 614 of the radially opposite second phase module 610-2 andfifth phase module 610-5 are connected in parallel to the second phaseterminal V. Respective first terminals of the coils 614 of the radiallyopposite third phase module 610-3 and sixth phase module 610-6 areconnected in parallel to the third phase terminal W. Respective secondterminals of the first phase module 610-1, second phase module 610-2,and third phase module 610-3 are connected at a first star point 616-1.Respective second terminals of the fourth phase module 610-4, fifthphase module 610-5, and sixth phase module 610-6 are connected at asecond star point 616-2. Thus, in this example, the phases are connectedin a star configuration. In other motors in accordance with the presentdisclosure, the phases may be connected in a delta configuration.

FIG. 10A also shows the measurement of the voltage between the two starpoints 616-1, 616-2. A difference in the voltage between the star pointsmay be used to diagnose a fault in the stator coils 614.

FIG. 10B is a hybrid diagram combining the axial end view of the stator61 (FIG. 9A) and the circuit diagram of the stator 61 (FIG. 10A). Aswell as showing the connection of the coils 614 together, and to thephase connections U,V, W and the star points 616-1, 616-2, FIG. 10Bshows the coils 614 in schematic form. FIG. 10B shows that the coils 614have end windings 617 at circumferential ends, respectively, whichallows the current to reverse direction between the inner coil portion614 a and the outer coil portion 614 b. FIG. 10B also shows that eachcoil 614 has multiple winding turns 6140. Although only two turns areillustrated, in practice, each coil may have more than two turns.

FIG. 11 shows the motor 60 in cross-section. While the previous figureshave only illustrated certain active parts of their respective motors,FIG. 11 also shows various other features, including features of an EPUin which the motor may be integrated.

The motor 60 includes a main motor housing 601 that includes, amongstother things, the stator 61. Various components of the stator 61 arevisible and labelled in FIG. 11 . This includes, for the two of the sixphase modules 610-1 to 610-6 that are visible in the cross-section ofFIG. 11 , the first, radially inner set of flux guiding stator elements612 a, the second, radially outer set of flux guiding stator elements612 b, the first, radially inner slot portion 613 a defined by the firstset of stator elements 612 a, and the second, radially outer slotportion 613 b defined by the second set of stator elements 612 b. Thecoil portions 614 a, 614 b are omitted from FIG. 11 to more clearly showthe slots 613 (e.g., the winding space).

The rotor 62 includes a rotor housing 625 mechanically coupled to theEPU drive shaft 630 via a coupling structure 626 that, for example, maybe disk-shaped. Thus, in this example, the rotor housing 625 rotateswith the drive shaft 630 about an axis of rotation 63. The active partsof the rotor (e.g., the permanent magnets that interact with the activeparts of the stator 61) are located within the rotor housing 625 andalso rotate together with the housing 625.

The permanent magnets include four groups of permanent magnets 622 a-1,622 a-2, 622 b-1, 622 b-2, each of which are circumferentiallydistributed around the rotor 62. In one example, each group of permanentmagnets 622 a-1, 622 a-2, 622 b-1, 622 b-2 is arranged as a Halbacharray. The first group of permanent magnets 622 a-1 and the second groupof permanent magnets 622 a-2 form a first set of magnets 622 a thatinteract with the magnetic field associated with the first, radiallyinner coil portion 614 a and the first set of flux guiding statorelements 612 a. The third group of permanent magnets 622 b-1 and thefourth group of permanent magnets 622 b-2 form a second set of magnets622 b that interact with the magnetic field associated with the second,radially outer coil portion 614 b and the second set of flux guidingstator elements 612 b.

The first group of magnets 622 a-1 is located axially adjacent to (e.g.,axially above) and facing a first axial end of the radially innerportions 612 a, 613 a, 614 a of the active parts of the stator 61. Thefirst group of magnets 622 a-1 is separated from the first axial end ofthe active parts of the stator 61 by a first axial air gap,schematically indicated in FIG. 11 but too small to see, having a widthG_(Air,1) in the axial direction. The second group of magnets 622 a-2 islocated axially adjacent to (e.g., axially below) and facing a secondaxial end of the radially inner portions 612 a, 613 a, 614 a of theactive part of the stator 61. The second group of magnets 622 a-2 isseparated from the second axial end of the active parts of the stator 61by a second axial air gap, schematically indicated in FIG. 11 but toosmall to see, having a width G_(Air,2) in the axial direction. Thevalues of G_(Air,1) and G_(Air,2) may be the same to balance loading.

The third group of magnets 622 b-1 is located axially adjacent to (e.g.,axially above) and facing a first axial side of the radially outerportions 612 b, 613 b, 614 b of the active parts of the stator 61. Thethird group of magnets 622 b-1 is separated from the first axial end ofthe active parts of the stator 61 by a third axial air gap,schematically indicated in FIG. 11 but too small to see, having a widthG_(Air,3) in the axial direction. The fourth group of magnets 622 b-2 islocated axially adjacent to (e.g., axially below) and facing a secondaxial end of the radially outer portions 612 b, 613 b, 614 b of theactive parts of the stator 61. The fourth group of magnets 622 b-2 isseparated from the second axial end of the active parts of the stator 61by a fourth axial air gap, schematically indicated in FIG. 11 but toosmall to see, having a width G_(Air,4) in the axial direction. Thevalues of G_(Air,3) and G_(Air,4) may be the same to balance loading,and may be the same as G_(Air,1) and G_(Air,2).

In use, the stator coils 614 of the phase modules 610-1 to 610-6 areexcited with current from inverter circuits. The current flows in acircumferential direction through the inner coil portion 614 a and theouter coil portion 614 b of the phase modules 610-1 to 610-6, changingdirection in the end windings 617. The magnetic flux generated by thecurrent is guided in magnetic circuits axially through the body portionsof the stator elements 612, radially through the projections of thestator elements 612, axially across the axial air gaps, andcircumferentially between rotor magnets 622. The magnetic field producedby the rotor magnets 622 interacts with the stator field to producetorque, which drives rotation of the rotor 62 and, via the couplingstructure 626, rotation of EPU drive shaft 630. Rotation of the driveshaft 630 drives rotation of a propeller or fan, and a propellerinterface 65 is shown in FIG. 11 .

Thus, a three-phase transverse flux motor 60 with axial air gaps and twocoils per phase has been described. For completeness, FIGS. 12A and 12Billustrate an equivalent transverse flux motor 70 with radial air gaps.It will be appreciated that a radial air gap transverse flux motor 300was described with reference to FIGS. 7-8 , but for a single phasehaving a single coil.

FIG. 12A is an axial end view of a transverse flux motor 70 having astator 71 and a rotor 72. The rotor 72 rotates about an axis of rotation73. Once again, the stator 71 includes six circumferentially arrangedphase modules 710-1 to 710-6. Radially opposite phase modules (e.g.,phase modules 710-1 and 710-4) are associated with the same phase (e.g.,phase U) and are connected, for example, as shown in FIG. 10A. The rotorincludes a radially inner rotor portion 72 a and a radially outer rotorportion 72 b, with the stator 71 positioned radially therebetween. Thestator coils 74 are omitted from FIG. 12A for clarity, but a singlestator coil 74 of one phase module 710 is illustrated in and will bedescribed with reference to FIG. 12B.

As in the motor 60 of FIGS. 9A-9B, 10A-10B, and 11 , each phase module710-1 to 710-6 of the motor 70 of FIGS. 12A-12B includes two sets ofstator elements 712 a, 712 b defining two circumferentially extendingslot portions 713 a, 713 b housing two circumferentially extending coilportions 714 a, 714 b of the coil 714. However, while the two sets ofstator elements 612 a, 612 b of the previously described motor 60 areradially spaced, the two sets of stator elements 712 a, 712 b of themotor 70 of the present example are axially spaced. This can be mosteasily appreciated from FIG. 12B, which shows one phase module 710. FIG.12B shows the two sets of axially spaced stator elements 712 a, 712 band the coil portions 714 a, 714 b, which are connected atcircumferential ends of the phase module 710 by end windings 717. Thedirection of current flow is indicated by the circumferential arrowslabelled “{right arrow over (I)}”. FIG. 12B shows that in this example,each coil 714 is formed of a plurality of winding turns.

In the present example, each set of stator elements 712 a, 712 bincludes a plurality of circumferentially arranged, evenly distributedflux guiding stator elements 712 (e.g., stator elements 712). Each ofthe stator element 712 is substantially as described above withreference to FIGS. 7-8 (e.g., each stator element 712 includes a bodyportion 7121 and two projections 7122, 7123). The stator elements 712are oriented so that the body portions 7121 extend radially and theprojections 7122, 7123 extend axially from the body portion 7121. Theprojections 7122, 7123 of circumferentially adjacent stator elements 712i, 712 ii of each set 712 a, 712 b alternately project axially inwardlyand axially outwardly so as to define an open slot cross-section,similar to that shown in FIG. 7B. Collectively, all of the statorelements 712 within a set (e.g., set 712 a) define a slot portion (e.g.,first axial slot portion 713 a).

Comparing FIG. 12A and FIG. 12B, the stator phase modules 710-1 to 710-6extend axially into the plane of the page such that only one axialportion (e.g., portion 712 a) of each phase module 710 is visible.Similarly, the two rotor portions 72 a, 72 b extend axially into theplane of the page. Each rotor portion 72 a, 72 b includes first andsecond axially spaced groups of permanent magnets such that, in total,the rotor 72 has four groups of magnets: a radially inner and axiallyinner first group 722 a-a, a radially inner and axially outer secondgroup 722 a-b, a radially outer and axially inner third group 722 b-a,and a radially outer and axially outer fourth group 722 b-b. The dashedlines used for labels 722 a-b and 722 b-b indicate that the permanentmagnets of the second group 722 a-b and the fourth group 722 b-b areaxially behind those of the first group 722 a-a and the third group 722b-a. Each group of permanent magnets 722 a-a, 722 a-b, 722 b-a, 722 b-bmay be arranged as a Halbach array.

The magnets of the first group 722 a-a face a radially inner side of thestator 71 at a first axial height and are separated from the radiallyinner side by a first radial air gap of width G_(Air) The magnets of thesecond group 722 a-b face the radially inner side of the stator 71 at asecond axial height and are separated from the radially inner side by asecond radial air gap of width G_(Air,2). The magnets of the third group722 b-a face a radially outer side of the stator 71 at the first axialheight and are separated from the radially outer side by a third radialair gap of width G_(Air,3). The magnets of the fourth group 722 b-b facethe radially outer side of the stator 71 at the second axial height andare separated from the radially outer side by a second radial air gap ofwidth G_(Air,4). The first radial air gap width G_(Air,1) and the secondradial air gap width G_(Air,2) may be the same to balance loading.Likewise, the third radial air gap width G_(Air,3) and the fourth radialair gap width G_(Air,4) may be the same to balance loading. In someexamples, all four radial air gaps widths are the same. The radial airgaps are only schematically indicated in FIGS. 12A-12B because the airgaps are too small to resolve.

In use, the stator coils 714 of the phase modules 710-1 to 710-6 areexcited with current from inverter circuits. The current flows in acircumferential direction through the axially inner portion 714 a andthe axially outer coil portion 714 b of the phase modules 710-1 to710-6, changing direction in the end windings 717. The magnetic fluxgenerated by the current is guided in magnetic circuits radially throughthe body portions of the stator elements 712, axially through theprojections of the stator elements 712, radially across the radial airgaps, and circumferentially between rotor magnets. The magnetic fieldproduced by the rotor magnets interacts with the stator field to producetorque that drives rotation of the rotor 72 a, 72 b.

Thus, a three-phase transverse flux motor 70 with radial air gaps andtwo coils per phase has been described.

As described above with reference to FIGS. 3 and 4 , for VTOLapplications, it may be desirable to utilize multi-lane (e.g.,dual-lane) electric motors. FIG. 13 and FIG. 14 illustrate how amulti-lane architecture may be implemented in a transverse flux electricmotor.

FIG. 13 illustrates a transverse flux motor 70′ (e.g., a motor) withradial air gaps and two independent power lanes (e.g., twosub-machines). The motor 70′, which is similar in its construction tothe motor 70 of FIGS. 12A-12B, includes six phase modules 710-1′ to710-6′. While the motor 70 of FIGS. 12A-12B has two coils per phase withradially opposite coils connected and belonging to the same phase, themotor 70′ has two three-phase sub-machines 70 a′, 70 b′ with one coilper phase, and radially opposite coils correspond to differentsub-machines 70 a′, 70 b′. Each sub-machine 70 a′, 70 b′ receives itspower from a different inverter so that if an inverter fails, onesub-machine 70 a′, 70 b′ is not affected by the failure.

In more detail, the circumference of a stator of the motor 70′ iscircumferentially divided into two sectors each spanning Π radians (180degrees): a first sector 70 a′ and a second sector 70 b′. The firstsector 70 a′ corresponds to a first three-phase sub-machine 70 a′ andhas three phase modules 710-1′ to 710-3′, each corresponding to onephase of the first sub-machine 70 a′. The second sector 70 b′corresponds to a second three-phase sub-machine 70 b′ and has threephase modules 710-4′ to 710-6′, each corresponding to one phase of thesecond sub-machine 70 b′. The stators of the sub-machines 70 a′, 70 b′share and interact with a common rotor, which is configured in a same orsimilar way to the dual rotor of the motor 70 of FIG. 12A.

By increasing the number of sectors into which the circumference isdivided, the number of sub-machines may be increased. For a number ofsub-machines equal to N_(L), there may be N_(L) sectors each spanning2Π/N_(L) radians (360/N_(L) degrees). The number of coils per phase maybe increased by increasing the number of phase modules per sector.

FIG. 14 illustrates a transverse flux motor 60′ with axial air gaps andtwo independent power lanes (e.g., two sub-machines). The twosub-machines 60 a′ and 60 b′ are implemented by axially stacking twosets of active parts. Specifically, the motor 60′ has two axiallystacked sub-machines 60 a′, 60 b′, each of which is of similarconstruction to and operates in much the same way as the axial air gapmotor 60 of FIGS. 9A-9B, 10A-10B and 11 . Rotors of the two sub-machines60 a′, 60 b′, each of which is of dual-rotor construction, aremechanically coupled so that the rotors rotate together, though it willbe appreciated the two rotors may instead be independent and beseparately connected to an output drive shaft.

An advantage of the axial stacking approach is that, as well asimplementing multiple sub-machines for increased fault tolerance, thetorque developed by the motor 60′ is increased without requiring anincrease in the motor diameter or the slot current density. Although theactive parts mass does increase, the use of some common features (e.g.,non-active features such as cooling and support structures) limits theoverall increase in the mass of the motor 60′. The number of power lanesmay be increased beyond two, if this is desired, by axially stackingmore than two sub-machines and/or dividing the circumference of eachstator into multiple sub-machines as shown in FIG. 13 .

Motors in accordance with the present disclosure may be configured tohave particularly high active part torque densities, defined in Equation(1). For example, motors may have a value of p_(act) of at least 50Nmkg⁻¹. Table 3 illustrates the calculation of p_(act) for three motorsin accordance with the present disclosure, each of which is a transverseflux motor.

TABLE 3 m_(act) τ_(peak) ρ_(act) (kg) (Nm) (Nmkg⁻¹) 10.2  870 85.2 13.41300 97.0 17.8 1450 81.5

As shown, each of the example transverse flux motors has a particularlyhigh value of p_(act), in excess of 80 Nmkg⁻¹. Noting that a VTOLaircraft may include at least four EPUs, such a high value of p_(act)results in a significant mass saving when compared even to VTOL aircraftutilizing optimized radial flux motors.

The increased active parts torque density may be understood by comparingthe two-dimensional magnetic circuits of the radial flux motor (FIG. 5B)and the three-dimensional magnetic circuits of the transverse flux motor(FIG. 8 ). Referring first to FIG. 5B, the magnetic circuits aresubstantially two-dimensional (e.g., the magnetic circuits lie in planesperpendicular to the axis of rotation 230) and pass through the annularregion of the stator 210 in which the coils 214 are housed. There is,therefore, competition for space in this annular region between theconductor, which carries the current that generates the stator field,and the stator teeth that guide the flux in the radial direction. Thus,for a motor of a given active parts diameter, any gain in performancethat may be realized by increasing the conductor volume may be offset bythe effects of a corresponding reduction in a stator iron volume, andvice versa. For example, a higher conductor volume may allow more torqueto be produced, or the same torque to be produced at lower currentdensity, the latter reducing the cooling burden. However, this wouldrequire more slender stator teeth, which are less efficient flux guides,and/or fewer stator teeth, which may result in higher torque ripple(e.g., especially at the low rotor speeds for VTOL aircraft). Thus, animprovement in one performance metric (e.g., peak torque) will likelyrequire either a reduction in another performance metric (e.g.,efficiency and torque ripple) and/or an increase in the mass of theactive parts of the motor. Referring now to FIG. 8 , in contrast withFIG. 5B, the magnetic circuits are three-dimensional and spiral aroundthe annular winding space 313 that houses the conductor. There istherefore no competition or much more limited competition for space inthe stator between the conductor and the flux guiding stator iron.Consequently, both the stator pole design and number may be optimized(e.g., without reducing the conductor volume), resulting in a higheractive parts torque density.

In designing for a high value of p_(act), it is useful to introduce adimensionless machine parameter Γ, defined in Equation (5) as thecumulated volume of the conductor (e.g., the stator coils),V_(conductor), included in the motor divided by cumulated volume of theflux guiding iron material, V_(iron), included in the motor. Inaccordance with the present disclosure, a notably high value of F,greater than or equal to 0.25, may be selected to promote the productionof high torque with a low active parts mass. Table 4 shows values of Ffor three motors in accordance with the present disclosure and sized foran EPU of a VTOL aircraft:

TABLE 4 V_(conductor) V_(iron) (cm³) (cm³) Γ Example 1 39.2 62.2 0.63Example 2 34.1 92.2 0.37 Example 3 52.2 49.7 1.02

The iron material may be present in both the stator and the rotor.However, in the transverse flux motors of the examples described herein,only the stator includes iron material. This reduces the iron volume andpromotes a higher value of F.

Another characteristic parameter for the purposes of an EPU of a VTOLaircraft is A, defined in Equation (3) as the ratio of the active partstorque density and the slot current density at the peak rated current. Amay be a useful parameter for optimizing a motor for a VTOL EPU becausethe parameter rewards torque production but penalizes the addition ofactive parts mass, which increases the EPU weight, and at the same timepenalizes the use of a high slot current density, which creates onerouscooling requirements and increases the likelihood of failures. Inaccordance with the present disclosure, a particularly high value of A(e.g., greater than or equal to 5 ρN³mkg⁻¹A⁻¹) may be selected. Table 5shows values of A for three transverse flux motors in accordance withthe present disclosure:

TABLE 5 ρ_(act) J_(slot,peak) Λ (Nmkg⁻¹) (Amm⁻²) (μN³mkg⁻¹A⁻¹) 84 6 14108 7.5 12.5  76 11 6.9

As noted above, the radial flux motor 200 has magnetic circuits that aretwo-dimensional and pass radially through the annular region of thestator 210 in which the slots 213 are defined. This creates competitionfor space in the annular region of stator 210 between the flux guidingmaterial (e.g., the stator teeth 212) and the slot 213 that houses theconductor (e.g., the coils 214). This provides that increasing thenumber of stator pole pairs requires a decrease in the volume ofconductor. Equivalently, increasing the volume of conductor requires adecrease in the number of stator pole pairs. In contrast, in atransverse flux motor, there is no, or much more limited, competitionfor space in the annular stator region. Thus, the number of stator polepairs, formed by the stator elements 312 in this example, may beincreased with no impact on the volume of conductor. The impact of thismay be appreciated from FIG. 15 .

FIG. 15 is a plot 1100 illustrating how, for a motor of a givendiameter, the tangential force (y-axis) developed by the motor varieswith the pole pitch, P_(θ)(x-axis), and the air gap width, G_(Air). Twoplots 1101, 1102 corresponding to two air gap widths are shown: a 3 mmair gap (plot 1101) and a smaller 1.4 mm air gap (plot 1102). Thesevalues are shown purely for the purpose of explanation.

As can be seen from both plots 1101, 1102, at small values of the polepitch, P_(θ), the tangential force increases as the pole pitchincreases. However, the tangential force eventually reaches a maximum ata particular value of the pole pitch P_(θ,max). Increasing the polepitch beyond P_(θ,max) decreases the tangential force and thus reducesthe torque developed by the motor. By comparing the two plots 1101,1102, it is also shown that: (i) the tangential force increases as theair gap decreases; and (ii) the value of P_(θ,max) decreases as the airgap decreases.

From FIG. 15 , it may be appreciated that to increase the torque densityof a motor of a given diameter, it is desirable to decrease the air gapwidth. However, for a given air gap width, torque production may only bemaximized if the pole pitch may be decreased to P_(θ,max). In a radialflux motor, the extent to which P_(θ) may be decreased (e.g., byincreasing the number of pole pairs) is limited by competition for spacein the stator. In a transverse flux motor, however, the extent to whichP_(θ) may be decreased (e.g., by increasing the number of pole pairs)may instead only be limited by manufacturing constraints and the fluxguiding efficiency of the stator iron. Thus, a transverse flux motor mayaccess an upper-left region of the plot 1100 corresponding to hightorque density, whereas a radial flux motor may only access a relativelylower-right region of the plot.

In accordance with the present disclosure, to further optimize a motorfor use in an EPU of a VTOL aircraft, the value of a motor parameter γ,defined in Equation (9) as the product of the pole pitch, P_(θ), and theair gap width, G_(Air), may be selected to be in the range 5 to 100micro radian-metres. Table 6 shows examples of values of γ for threeexample motors in accordance with the present disclosure. Values areprovided in micro radian-meters.

TABLE 6 Υ (10⁻⁶ radian-meters) Example 1 Example 2 Example 3 11.0 26.768.1

For motors sized for VTOL aircraft, the selection of a value of γ inthis range may optimize the torque-producing tangential force and thusincrease the active parts torque density, p_(act). Small values ofP_(θ)(e.g., less than or equal to 10 degrees, or less than 5 degrees)and high values of the pole pair number (e.g., at least 15, or greaterthan or equal to 50) may be provided, along with small values of the airgap width (e.g., less than or equal to 1.5 mm). Motors in accordancewith the present disclosure may have more than one air gap because ofthe use a dual rotor design and/or the use of axial stacking of activeparts to implement multiple lanes. All air gaps of a given motor may beapproximately the same size, such that the value of γ will beapproximately the same for all air gaps of a motor. Where different airgaps are used, however, the largest air gap may be used to calculate γas the largest air gap may limit the torque density.

Motor-inverter combinations in accordance with the present disclosuremay also have optimized values of a parameter Π, defined in Equation(11). Π is the ratio of the pole arc length, P_(L) (see Equation (28))and the maximum value of the electrical frequency, f_(max), of thecurrent output by the inverter and received by the stator coils of themotor during use. In accordance with the present disclosure, the valueof Π may be between 1 and 30 μms, which is unusually low. Table 7 showsexamples of values in accordance with the present disclosure.

TABLE 7 P_(L) (mm) f_(max) (kHz) Π (μms) Example 1 7.0 1.5 4.7 Example 24.2 0.6 7.0 Example 3 17.5 1.2 14.6

As described above with reference to FIG. 4 , particularly in point d),the propulsion system of a VTOL aircraft has a large number of invertersas a result of its distributed propulsion system and fault tolerantelectrical architecture. This results in the stacking of inverter lossesand provides that inverter efficiency may have a significant impact onperformance (e.g., aircraft mission range). Selecting an unusually lowvalue for Π(e.g., in a range of 1 to 30 μms or in a range of 3 to 15μms) has been found to reduce inverter losses when operating at arelatively low rotor speed. The use of a low value of Π may thereforenot only reduce inverter losses, but also allow for the omission of aspeed-reducing gearbox in the EPU without sacrificing low aerodynamicnoise or motor efficiency.

An important consideration in the context of aerospace electricalmachines is fault tolerance. In accordance with the present disclosurein which the electrical machines may be the permanent magnet type, thetolerance to a stator terminal short circuit fault may be particularlyimportant.

In the event of a stator terminal short circuit fault (e.g., a shortcircuit fault condition in the electrical network connected to thestator terminals), the rotation of the rotor will drive a fault currentinto the network for as long as the rotor excites the stator windings.In motor designs that feature rotor windings, it is possible to stopexcitation of the rotor windings to prevent the excitation of a voltagein stator windings and thus stop the fault current. However, in apermanent magnet motor, the rotor is permanently excited and will,unless the permanent magnets are demagnetized or the rotor is moved awayfrom the stator, continue to excite a voltage in the stator windingsthat will drive the fault current. With zero or little impedance in theshort-circuited electrical network, this fault current may be verylarge. The heat dissipated by the stator windings, which causes heatingof the coil insulation, increases with the square of the current (|²Rlosses).

One potential mitigation to this problem is for the EPU to include amechanism or device to physically disconnect the permanent magnet rotorfrom the propeller fan so that the inertia of the propeller does notcontinue to force rotation of the rotor. For example, a freewheeltransmission may be included in the EPU. However, this solution may addmass, complexity, and maintenance requirements to the EPU. Anotherpotential mitigation would be to provide additional overrating to thecooling system of the motor, so that the cooling system may maintain thetemperature of the insulation at or below its rated temperature even inthe presence of a terminal short circuit fault. However, this also addsmass to the EPU and may make air cooling (described in more detailbelow) unfeasible, adding even more mass to the EPU due to therequirement to adopt liquid cooling.

In accordance with the present disclosure, an electrical machine mayhave a short-circuit insulation temperature parameter, ζ defined inEquation (17) that satisfies the inequality:

$\zeta = {\frac{\theta_{{ins},{cont}}( I_{SC} )}{\theta_{{ins},{cont}}( I_{cont} )} \leq {1.1.}}$

In the above equation, I_(Sc) is the steady-state short circuit current,and I_(cont) is the continuous rated current (e.g., the highest currentthe stator coils are rated to carry for a sustained period; this isassociated with production of the maximum continuation rated torque,τ_(max,cont)) θ_(ins,cont)(I_(SC)) is the temperature of the insulationwhen carrying the steady-state short circuit current, andθ_(ins, cont)(I_(cont)) is the temperature of the insulation whencarrying the continuous rated current. Designing a motor to have ashort-circuit insulation temperature parameter, ζ less than or equal to1.1 may allow the stator coils and their insulation to be sufficientlycooled following a terminal short circuit fault without additionaloverrating the cooling system. A value of ζ in the range of 0.7 to 1.0may be provided and may, for example, allow for the use of air coolingin a transverse flux motor without additional overrating of the coolingsystem or a reduction in the performance of the motor during normaloperation.

Additionally or alternatively, a short circuit current ratio, ζ definedin Equation (16), may satisfy the inequality:

${{0.5} \leq \xi} = {\frac{I_{SC}}{I_{peak}} \leq {1.2}}$

In the above equation, I_(peak) is the peak rated current (e.g., thecurrent associated with production of the peak rated torque, τ_(peak)) Avalue of this ratio in a range of 0.6 to 0.9 (e.g., in a transverse fluxmotor) may strike a good balance between fault tolerance and goodelectrical and mechanical performance.

A further motor design optimization in accordance with the presentdisclosure is to select a design with a value of a characteristic motorparameter Δ, defined in Equation (6), greater than or equal to 65Nmkg⁻¹. Table 8 shows the calculation of Δ for three exemplarytransverse flux motors, sized for use in the EPU of a VTOL aircraft.

TABLE 8 ρ_(act) (Nmkg⁻¹) cos(Ø) Δ (Nmkg⁻¹) Example 1 85 0.65 131 Example2 95 0.75 127 Example 3 74 0.85  87

The selection of a value of greater than or equal to 65 Nmkg⁻¹,particularly a value in a range of 80 to 190 Nmkg⁻¹, may provide asurprising combination of low EPU mass and fault tolerance. For example,such a selection may correspond to a sweet spot in the combined mass ofan EPU's motor, inverter, and cooling system while offering goodtolerance against stator terminal short circuit faults. This may beunderstood in terms of the effect of the power factor and itsrelationship with the torque density of the motor. A motor with a lowpower factor may require oversized power electronics but will also havea lower steady state terminal short circuit current. Thus, the selectionof the power factor affects the inverter mass and also the requiredcooling system mass, as the cooling system may be sized to cool themotor under short circuit conditions. At the same time, the value of thepower factor is mediated by the inductance of the motor, which dependson the quantity and distribution of active parts. This affects theactive parts mass and the peak rated torque. A value of Δ in a range of80 to 190 Nmkg⁻¹ may strike an effective balance between these competingrequirements.

It is also useful to introduce a motor parameter Z, defined in Equation(14) as the product of the power factor of the motor and active partsmass divided by the efficiency, η, of the motor. The efficiency isdefined as the efficiency when the motor is producing the maximumcontinuous rated torque, T_(max,cont), at ISA sea level conditions.Table 9 illustrates values of Z in accordance with the presentdisclosure. The values of Z are notably low and may be associated with astrong balance between efficiency and fault tolerance in a motor sizedfor VTOL aircraft.

TABLE 9 Z (kg) Example 1 Example 2 Example 3 11 7.2 14.5

In accordance with the present disclosure, the value of Z may be lessthan or equal to 30 kg or in a range of 5 to 15 kg. This may be achievedmost effectively in a transverse flux motor, where the inductance may betuned to achieve a desirable power factor, (e.g., in a range of 0.6 to0.9), without a significant negative impact on the efficiency of themotor. In a radial flux motor, the length of the magnetic circuits(illustrated in FIG. 5B) may be shorter than those in a similarly-sizedtransverse flux motor (illustrated in FIG. 8 ). The tuning of theinductance of the radial flux motor may therefore require the additionof significant active parts mass, or the selection of a sub-optimaldesign (e.g., the selection of long and narrow stator teeth; see FIG. 6, design (i)), which may increase the inductance but at the same timereduce the efficiency of the motor.

In a similar manner, a value of a motor parameter λ, defined in Equation(20) as the product of the efficiency and the inductance of the machinedivided by its active parts mass, may be tuned to improve balancebetween efficiency and fault tolerance. Table 10 illustrates values of μin accordance with the present disclosure. The values of λ are notablyhigh.

TABLE 10 λ (μHkg⁻¹) Example 1 Example 2 Example 3 6.8 1.8 3.0

The value of λ may be selected to be greater than or equal to 1.4μHkg⁻¹, while values in the range of 2.1 to 5.5 μHkg⁻¹ may provide aparticularly good balance between the competing constraints. The machineinductance itself, L_(machine), may be relatively high, especiallyrelative to the mass of the active parts (i.e., L_(machine) divided bym_(act) may be particularly high).

Another important consideration in the design of a motor for an EPU ofan aircraft is the capability of the cooling system of the motor. Thecooling system may be capable, at all relevant operating conditions, ofremoving heat from the motor at a rate sufficient to keep the motorbelow a rated temperature. Herein, the rated temperature may be amaximum rated temperature of the coil insulation, θ_(ins,max) Thecooling system may significantly add to the mass of the EPU, and thisadditional cooling system mass (m_(cool)) is multiplied by the number ofEPUs on the aircraft. Thus, rather than designing an EPU with a hightorque production capability and an aggressive cooling system, which mayhave a high mass, it may be desirable to consider a parameter ∇, definedin Equation (12):

$\begin{matrix}{\nabla = \frac{\tau_{\max,{cont}}}{m_{act} \times C_{\max,{cont}}}} & (12)\end{matrix}$

In this equation, τ_(max,cont) is the maximum continuous rated torque,and C_(max,cont) is the heat capacity cooling rate required to maintainthe coil insulation at or below its rated temperature, θ_(ins,cont)assuming operation at ISA sea level conditions. C_(max,cont) may bedefined as the product of the specific heat capacity of a coolant of thecooling system (at ISA sea level conditions) multiplied by the mass flowrate of the coolant required to maintain the coil insulation at or belowθ_(ins,max) In accordance with the present disclosure, a value of ∇ maybe selected so that the combined mass of the active parts and thecooling system may be optimized relative to the torque producingcapability of the motor. Table 11 illustrates values of ∇ for a motorsized for an EPU of an aircraft in accordance with the presentdisclosure. The values of ∇, which are notably high, are quoted in unitsof Kskg⁻¹ (Kelvin-seconds-per-kg):

TABLE 11 ∇ (Kskg⁻¹) Example 1 Example 2 Example 3 0.19 0.27 0.55

The value of ∇ may be selected to be greater than or equal to 0.1Kskg⁻¹, greater than or equal to 0.18 Kskg⁻¹, or greater than or equalto 0.21 Kskg⁻¹.

It is also useful to introduce a dimensionless figure of merit, F, for amotor of a VTOL aircraft. F is defined in Equation (22):

$\begin{matrix}{F = {\frac{\tau_{\max,{cont}}}{m_{act}}\frac{p_{{air},0}}{C_{p}{{\overset{.}{m}}_{\max,{cont}}( {\theta_{{ins},\max} - \theta_{{air},0}} )}}\frac{2\pi \times D_{ref}}{\omega_{{mech},{cont}}}( \frac{D_{ref}}{D_{act}} )^{2}}} & (22)\end{matrix}$

In this equation, τ_(max, cont) is the maximum continuous rated torque,mast is the active parts mass, C_(p) is the specific heat capacity ofthe coolant at ISA sea level conditions, θ_(ins,max) is the maximumrated temperature of the insulation for operation at the maximumcontinuous rated torque, {dot over (m)}_(max,cont) is the mass flow rateof the coolant required to maintain the insulation at or belowθ_(ins,max) during ISA sea level operation at τ_(max, cont),ω_(mech,cont) is the angular speed of rotation (in radians per second)of the rotor of the motor while producing the maximum continuous ratedtorque, and D_(act) is the active parts diameter. The remaining valuesare fixed, nominal operational, values: P_(air,0) is a nominal ambientair pressure equal to 100 kPa, ° airs) is a nominal ambient airtemperature of 318 Kelvin, and D_(ref) is a nominal motor diameter setequal to 0.5 meters.

For the purposes of comparing two motors, any value may be selected forD_(ref) as long as the same value of D_(ref) is used for bothcalculations. The value of F is decreased by using an active partsdiameter, D_(act) greater than D_(ref) but increased by using an activeparts diameter, D_(act), less than D_(ref). In other words, the equationfor F penalizes the use of an arbitrarily large active parts diameter tomeet the torque and speed requirements of the motor, as the use of anarbitrarily large diameter would create installation and aerodynamicdrag issues. The selection of 0.5 meters for D_(ref) reflects that 0.5meters is a reasonable value for certain EPU designs. If calculating andcomparing values of F for a smaller platform (e.g., an unmanned aerialvehicle (UAV) or drone), a smaller value of D_(ref) may be selected(e.g., 0.1 meters). If calculating and comparing values of F for alarger platform (e.g., a larger aircraft), a higher value of D_(ref) maybe selected (e.g., 1.0 meters). Accordingly, in Equation (22), it is thevalue of D_(act), and not D_(ref), that characterizes the motor.

In order to provide a particularly good balance between the competingrequirements of physical size (e.g., active parts diameter), mass,torque production, and cooling, electrical machines in accordance withthe present disclosure may have a particularly high value of F. Thefirst two rows of Table 12 illustrate values of F for motors inaccordance with the present disclosure. For comparison, the third lineof Table 12 illustrates the value of F for an exemplary radial fluxmotor designed for use in a CTOL aircraft having a more conventionalvalue of F.

TABLE 12 F Air-cooled transverse flux motor (VTOL) 5.2 Liquid-cooledradial flux motor (VTOL) 2.3 Liquid-cooled radial flux motor (CTOL) 0.3

According to the present disclosure, the value of F may be greater thanor equal to 1.9. In some examples of electric motors for EPUs of VTOLaircraft, particularly those utilizing a transverse flux arrangement,the value of F may be greater than or equal to 2.5.

A motor and EPU for these applications may use an air cooling system.This is partly due to a reduction in the complexity and maintenancerequirements associated with a liquid cooling system. However, apotentially more significant benefit is the reduction in the cumulatedmass of the components of the cooling system, which may otherwise make asubstantial contribution to the EPU mass and platform mass. For example,a liquid cooling system will include not only the mass of the liquidcoolant, but may also include: the mass of the coolant tank; conduits(e.g., piping) through which the coolant flows; the mass of pumps,valves, and other fluid flow modulating components; the mass of filters;and the mass of heat exchangers. In one example, the mass of a liquidcooling system sized for a motor of a VTOL aircraft EPU is about 14 kg,representing approximately 20-25% of the overall mass of the motor. Ifeach one of the six EPUs of the exemplary VTOL aircraft 1 of FIG. 1 hadsuch a cooling system, the total motor cooling system mass for theentire aircraft would be about 84 kg, which is on the order of the massof a passenger. In contrast, an example of an air-cooling system havinga mass generally limited to the mass of filter components and flowdirecting components may only be about 3-5 kg per EPU.

While the advantages of selecting an air-cooling system may be clear,implementing an air-cooling system in a motor for an EPU of a VTOLaircraft requires more consideration. Air has a relatively low specificheat capacity compared with certain liquid coolants (e.g., 1006 Jkg⁻¹K⁻¹for air, compared with 1745 Jkg⁻¹K⁻¹ for one oil-based coolant), and theavailable mass flow rate may be limited in VTOL applications due to boththe low density of air compared to liquid and the relatively slowmovement of the aircraft at some operating points. This may limit therate at which heat may be removed from the motor. If the slot currentdensity, J_(slot), is high, there will be high resistive losses (|²Rlosses) and/or a lack of free space in the slot to effectively cool thecoils, which may make air-cooling impractical. If the slot currentdensity, J_(slot), is too low, the motor may not be able to meet itstorque production requirements.

In accordance with the present disclosure, the selection of a transverseflux motor with one or more of the optimizations described above (e.g.,an optimized value of γ to access the peak of the torque curveillustrated in FIG. 15 ) may allow for use of air cooling in an EPU of aVTOL aircraft, particularly where the flow of cooling air is supplied todirectly contact the coils. FIGS. 16 to 22 illustrate transverse fluxelectrical machines with air cooling systems and, more specifically,transverse flux motors with directly cooled conductors.

FIG. 16A and FIG. 16B show a motor 60 in perspective view. The motor 60includes a rotor 62 that is coupled to a drive shaft 630 via a couplingstructure 626 and rotates about an axis of rotation 63. The motor 60further includes a bearing unit 64. Reinforcement ribs 670 are arrangedcircumferentially around the bearing unit 64 and the axis of rotation 63and are fixed to the bearing unit 64 and to a base plate 672 of thebearing unit 64. The coupling structure 626 is visible in theperspective bottom view of FIG. 16B. At a radially outer region 674 ofthe coupling structure 626, the coupling structure 626 is connected tothe rotor 62. At a radially inner region 676 of the coupling structure626, the coupling structure 626 is connected to the EPU drive shaft 630.

FIG. 17 shows the motor 60 of FIGS. 16A and 16B in cross-section. FIG.17 shows the same motor 60 as FIG. 11 , but further shows coolingchannels 602 in the stator 61 and omits the active parts for clarity. Anempty volume 662 that would accommodate the active parts is labelled, asare the locations of axial air gaps 615 formed between the active partsof the stator 61 and the rotor 62. In operation, an external flow ofambient air that impinges on the EPU due to, for example, movement ofthe aircraft and/or wind enters the motor housing and is guided by thecooling channels 602 in a radially outward direction into the volume 662where the active parts are located. Thus, the ambient air directlycontacts and cools the active parts, including the stator coils.

For effective direct cooling, the volume 662 may not be completelyfilled and leaves space through which the cooling air may pass. Forexample, the stator coils 614 may define an effective cooling surfacearea that is directly exposed to air. FIGS. 18-20 illustrate an exampleof a stator phase module structure 610 by which the stator coils definean effective cooling surface area for direct cooling.

FIG. 18A and FIG. 18B show a stator phase module 610 mounted to itssupport structure 640. The phase module 610 is comparable to the oneshown in FIG. 9B, which is referred to for a detailed explanation. FIG.18A and FIG. 18B show the same embodiment; however, FIG. 18A is cut in aradial plane, enabling a view of the cross section of the first coilportion 614 a and the second coil portion 614 b of the coil 614, and thefirst slot portion 613 a and the second slot portion 613 b of the slot613. FIG. 18B shows the end windings 617.

The stator according to FIG. 18A includes an assembly 680 that may be amodular (e.g., prefabricated) component. The assembly 680 extends in theradial direction (r) and in the circumferential direction ((p), andincludes two axially spaced, non-magnetic and non-magnetizable supportstructures 640 i, 640 ii. These have radially inner fastening areas 690,692, at which the support structures 640 i, 640 ii may be connected to aplurality of ribs of the stator. This is done, for example, viaretaining projections of the ribs, as will be described with referenceto FIG. 21 .

Flux guiding stator elements 612 extend between the support structures640 i, 640 ii, and collectively provide the flux guiding stator iron 611of the stator. The stator elements 612 define the slots 613 (i.e., thewinding space) extending in the circumferential direction, in which thecoil 614 extending in the circumferential direction is arranged.

According to the present example, a flow of air (e.g., the flow ofexternal ambient air that enters the EPU and is directed by the statorcooling channels 602 of FIG. 17 ) flows radially through the assemblies680 in the region between the two support structures 640 i, 640 ii andflows across the stator elements 612 and the coil 614. This flow of aircools the stator elements 612 and the coil 614. The stator elements 612are each aligned radially. The stator elements 612 each have tworadially aligned side surfaces 694, 696 spaced apart in thecircumferential direction, both of which are cooled by a cooling airflow.

The coil 614 includes multiple individual winding turns 6140 (see FIG.18A) that are formed from a continuous winding wire. Further, each coilportion 614 a, 614 b of the coil 614 includes two axially spaced windingpackages: the first coil 614 a portion has a first axially spacedwinding package 614 a-i and a second axially spaced winding package 614a-ii in the first slot portion 613 a, and the second coil portion 614 bhas a third axially spaced winding package 614 b-i and a fourth axiallyspaced winding package 614 b-ii in the second slot portion 613 b. Thewinding packages 614 a-i, 614 a-ii 614 b-ii each have sections extendinglongitudinally in the circumferential direction of the coil 614. Asshown in FIG. 18B, through the deflected coil section that forms the endwinding 617, the winding packages 614 a-i, 614 a-ii, 614 b-i, 614 b-iiform a coil 614. A corresponding end winding 617 is found at the otherend of the coil 614.

The winding packages of each pair of winding packages 614 a-i, 614 a-ii,and 614 b-i, 614 b-ii are spaced apart in the axial direction from oneanother and from the support structures 640 i, 640 ii. In this way,cooling air may flow around the winding packages on their upper side andon their lower side. This is illustrated in FIG. 19A. The assembly 680defines three radially extending and axially spaced cooling air flowpassages A1, A2, A3 for cooling the winding packages 614 a-i, 614 a-ii,614 b-i, 614 b-ii. A first cooling air flow passage A1 runs adjacent tothe upper support structure 640 i, a second cooling air flow passage A2runs in an area between the winding packages 614 a-i, 614 b-i and thewinding packages 614 a-ii, 614 b-ii, and a third cooling air flowpassage A3 runs adjacent to the lower support structure 640 ii. Thedivision of the coil 614 into axially spaced winding packages 614 a-i,614 a-ii, 614 b-i, 614 b-ii increases the surface area of the windingthat is available for direct cooling. While two axially spaced windingspackets per winding portion are shown in this example, more than twoaxially spaced winding packets may also be provided. Alternatively, itis also possible that only one winding package is arranged in each slotportion 613 a, 613 b.

According to FIG. 18A and FIG. 19A, two winding packets 614 a-i, 614a-ii and 614 b-i, 614 b-ii, respectively, (or, e.g., one coil portion614 a, 614 b) may each be fixed by a fixing material 6130 in therespective slot portion 613 a, 613 b. In the present example, the fixingmaterial 6130 only slightly extends in the circumferential direction(e.g., being in the shape of a disk or plate) so as not to impaircooling by obstructing the cooling air flow. The fixing material may bearranged radially in front of or behind a stator element 612, so as tolimit a reduction in the cross-section facing and exposed to a radialair flow.

To avoid physical contact of the coil 614 with the stator elements 612,a mechanical protective layer may also be applied to the stator elements612 on the side facing the slot 613 (e.g., the slot portions 613 a, 613b). For example, an aram id paper may be used, analogous to the use ofslot papers in the slots of radial flux machines.

Referring again to FIG. 18A, the arrangement of the stator elements 612for defining the slot portions 613 a, 613 b is shown. The statorelements 612 are arranged in four circumferential rows: two rows 612a-i, 612 a-ii being of the first set 612 a of stator elements 612 anddefining the first slot portion 613 a, and two rows 612 b-i, 612 b-ii,being of the second set 6122 b of stator elements 612 and defining thesecond slot portion 613 b.

The stator elements 612 are, like those shown in previous examples,curved and/or bent. For example, the stator elements 612 may beclaw-shaped and/or curved in a C-shape. The stator elements 612 of therespective radially inner rows 612 a-i, 612 b-ii are concave, viewedfrom the radially outer side, and the stator elements of the respectiveradially outer rows 612 a-ii, 612 b-ii are convex, viewed from theradially outer side, so that their mutually facing sections togetherdefine the slot portions 613 a, 613 b. The stator elements 612 of eachof the two rows delimit the slot portions 613 a, 613 b transversely tothe circumferential direction. For this purpose, each stator element 612of a given row (e.g., row 612 a-i) forms a pair of stator elements witha circumferentially adjacent stator element belonging to a radiallyadjacent row (e.g. 612 a-ii) of the same set of stator elements (e.g.,612 a), and stator elements of a pair are oriented such that the statorelements of the pair oppose each other.

End portions (e.g., projections) of the stator elements 612 form poleheads (e.g., upper pole heads and lower pole heads; see, e.g., the poleheads 3122, 3123 in FIG. 7B). The end portions are positioned adjacentthe permanent magnets of the rotor 62 and are separated from thepermanent magnets of the rotor 62 only by an air gap (e.g.,corresponding to the air gap 615 of FIG. 17 ). For this purpose, it isprovided that the end portions or pole heads are each arranged in one ofthe support structures 640 i, 640 ii and terminate flush with theirouter sides 641 i, 641 ii. Accordingly, the upper end portions of thestator elements 612 lie in the outer plane of the upper outer side 641 iof the upper support structure 640 i, as shown in FIG. 18A.

A motor includes a plurality of the assemblies 680, adjoining oneanother in the circumferential direction. For example, six assemblies680 may be provided for the motor described with reference to FIGS. 9-11, with two per phase.

FIG. 19B is a schematic sectional view of an embodiment of a coil 614.The coil 614 has a continuous winding wire that is wound in a number ofwinding turns 6140, with each winding turn 6140 extending over an angleof 360°. A total of fourteen winding turns 6140 are provided in theexemplary embodiment considered. The coil 614 is configured such thatthe total of fourteen winding turns 6140 are arranged in four levels orcoil layers L1, L2, L3, L4, with three winding turns 6140-1 to 6140-3arranged in the first coil layer L1, four winding turns 6140-4 to 6140-7arranged in the second coil layer L2, four winding turns 6140-8 to6140-11 arranged in the third coil layer L3, and three coil turns6140-12 to 6140-14 arranged in the fourth coil layer L4. In theembodiment shown, each coil layer L1, L2, L3, L4 is arranged in an axialplane, parallel to each other.

The winding order is indicated by the arrows 6142. From the windingsequence, it follows that in the case of the winding turns 6140 of thefirst coil layer L1, a turn diameter D_(Turn) of the winding turns 6140decreases as the number of winding turns increases. In other words, thecontinuous winding wire or conductor is moving inwards with everywinding turn 6140 in the first coil layer L1. Thus, winding turn 6140-1has a larger turn diameter than winding turn 6140-2, which has a largerturn diameter than winding turn 6140-3. “Turn diameter,” in thiscontext, refers to the average diameter of a 360° loop of one windingturn 6140 around winding turn axis “W’, and not to the diameter of thewire or conductor. An example of the turn diameter D_(Turn) is shown inFIG. 19B for the twelfth winding turn 6140-12. A winding turn with asmaller turn diameter lies radially (e.g., with respect to the windingturn axis) within an adjacent winding turn with a larger diameter. Forexample, winding turn 6140-2 lies within winding turn 6140-1.

In contrast, in the coil layer L2, the turn diameter of the windingturns 6140 increases with an increasing number of winding turns. Forexample, the winding turn 6140-5 has a larger turn diameter than thewinding turn 6140-4. In the third coil level L3, the turn diameter ofwinding turns 6140 decreases again as the number of windings increases,and in the fourth coil level L4 the turn diameter increases again.

The described coil 614 forms winding packages 614 a-i, 614 a-ii, 614b-i, 614 b-ii corresponding to the winding packages 614 a-i, 614 a-ii,614 b-i, 614 b-ii of FIGS. 18A and 19A.

FIG. 20A shows an embodiment of a coil 614 with a structure according toFIGS. 19A-B in a view from above. FIG. 20A shows that the coil 614 mayhave a curved shape similar to that of a banana. Accordingly, the coil614 includes longitudinally extending sections 614 a, 614 b that may beconcavely bent. The longitudinally extending sections 614 a, 614 b arebent over and form deflected sections at the end windings 617. The topview of FIG. 20A shows the coil turns 6140-1, 6140-2, and 6140-3 of thefirst coil layer L1 of FIG. 19 .

FIG. 20B schematically shows a coil 614 that corresponds to FIGS. 19Aand 20A in terms of structure. The side view of FIG. 20B shows theindividual coil layers L1, L2, L3, and L4 in which a plurality ofwinding turns 6140 are formed. Further, the coil 614 of FIG. 20B isadditionally shown with fixing material 6130 in the form of fixing disks6132 that correspond to the fixing material 6130 of FIGS. 19A and 20A,and serve to arrange and position the coil 614 in the slot 613.

The shape of the coil 614 in FIGS. 19A-19B and 20A-20B is a non-limitingexample. In principle, the coil 614 (and/or the winding packages 614a-i, 614 a-ii 614 b-ii) in the winding diagram and structure shown inFIG. 19B may have other shapes, (e.g., circular, elliptical, or with aplurality of concave and convex areas). The use of fixing disks 6132according to FIGS. 20A, 20B is also optional.

As explained above with reference to FIG. 17 , ambient air may enter amotor and be directed radially outwardly by channels 602 towards theactive parts of the motor. As explained with reference to FIGS. 18A-B,19A-B, and 20A-B, the stator phase modules may be arranged so that thereare gaps (e.g., radial gaps between stator elements 612 and passages A1,A2, A3) for air flow for effective direct air cooling. FIGS. 21 and 22illustrate this in more detail for a dual-lane transverse flux motor 80.

FIG. 21 shows an electric drive unit with the motor 80. In FIGS. 11,16A-B, and 21, like reference numerals label like parts. The motor 80includes a rotor 81, a stator 82, an axis of rotation 83, and a bearingunit 84. The bearing unit 84 includes an axially arranged, rotatable EPUdrive shaft 830 and a static bearing part 822 that supports the EPUdrive shaft 830. The coupling structure described with reference toFIGS. 16A and 16B is not shown in FIG. 21 , but may be included in acorresponding manner. FIG. 21 shows a stator 82 as a ring structure witha large number of ribs 820 that adjoin one another in thecircumferential direction and each form a cooling air passage 821between the ribs 820. The active components of the stator 82 are heldand positioned by the ribs 820. For this purpose, the ribs 820 haveretaining projections 823.

A difference from FIGS. 11, 16A and 16B results from the fact that themotor unit of FIG. 21 includes two rotor-stator assemblies 8110, 8120,each forming a sub-machine of the dual-lane motor, which are axiallystacked (e.g., arranged one behind the other in the axial direction) andare fixed to one another. Accordingly, the rotor 81 includes threeaxially spaced outer walls 811, 812, 814, each of which has orintegrates permanent magnets 85, and two radially outer end walls 813,815. The outer walls 811, 812, 814 and the end walls 813, 815 form twoaxially spaced volumes 882 of the two rotor-stator assemblies 8110,8120, each containing the active components of the stator 82 of therespective assembly.

The permanent magnets 85 of the rotor are only shown on the right-handside of FIG. 21 for the sake of clarity. The permanent magnets 85 arearranged on the insides of the outer walls 811, 812, 814. The air gap615 shown in FIG. 17A runs between the permanent magnets 85 and theassociated stator poles of the assembly.

FIG. 21 also shows how a cooling air flow may be provided through thecooling air passages 821 and the active components of the stator 82arranged in the volume 882. The transverse flux machine may have a firstend 8010 facing a mechanical load to be driven (e.g., a propeller) and asecond end 8020 facing away from the load to be driven. In FIG. 21 , thetransverse flux machine forms openings 801 at its first end 8010, whichenable an air flow 860 to enter the motor unit in an initially primarilyaxial orientation. This may be supported by a fan 891, which is,however, optional. For example, the airflow may come from a propellerdriven by the EPU drive shaft 830.

The second end 8020 facing away from the load to be driven ishermetically sealed to prevent inflowing air from leaving the motor unitagain in the axial direction. For this purpose, a cover plate 802 isprovided, which is shown schematically. The cover plate 802 is connectedto the stator 82 in FIG. 21 , but may alternatively be connected to therotor 81, or be formed by a coupling structure similar to the couplingstructure 626 of FIGS. 11, 16A or the like, depending on the design.

By the cover plate 802, the inflowing air flow 860 flows radiallyoutwards as an air flow 861 through the cooling air passages 821 and theactive components of the stator arranged in the volume 882. The radialair flow 861 may also be optionally supported by fans 892.

The end walls 813, 815 of the rotor 81 are provided with radial openings816 that enable the cooling air flow 861 to be directed into theenvironment. Alternatively, openings may be formed in the motor unit atthe second end 8020 facing away from the load to be driven, while thefirst end 8010 facing the load to be driven may be sealed airtight inthis case. A further alternative is that a cooling air flow is directedradially inwardly through the stator 82. For this purpose, an air flowlocated at the outer circumference of the rotor, which may originatefrom a propeller, for example, may be deflected by baffles or otherdeflecting mechanisms or devices, and guided through openings 816 in thewalls 813, 815 of the rotor 81 into the stator 82. In this way, theradial direction of the air flow may be reversed, with the air flowgoing from radially outside to radially inside through the activecomponents of the stator (e.g., that are arranged in the volume 882) andthe cooling air passages 821.

FIG. 22 shows a detail cutaway and perspective view of the example motor80 shown in FIG. 21 . The first rotor-stator assembly 8110 and thesecond rotor-stator assembly 8120 (e.g., sub-machines) are visible. Theconnection between the radially inner fastening areas 690, 692 of statorassemblies 680 and retaining projections 823 of the stator 82 (e.g.,integrally connected to the ribs 820, respectively) is also visible.Further, the axial air gaps 815, which are arranged between thepermanent magnets 85 of the rotor 82 and each support structure 640 i,640 ii, are indicated.

As noted above, for the purposes of aircraft (e.g., VTOL aircraft), anair cooling system may be used because of the associated reduction inEPU mass, complexity, and maintenance requirements. However, meeting theplatform torque production requirements (e.g., with a high active partstorque density) while using air cooling may necessitate the use ofdirect air cooling (e.g., a cooling system in which heat is transferreddirectly from the coils into the cooling air, rather than via a heatexchanger). FIGS. 16 to 22 show motors 60, 80, stator modules 680, andcoils 614 that incorporate spaces through which air may flow to directlycool the active parts. For example, the illustrated coils have multiple(e.g., two) winding packages to define an intermediate air passage A2for direct cooling of axial inner regions of the coil, with additionalspaces left in the axial outer regions to define axial outer airchannels A1, A3.

Increasing the amount of free space in the active parts region (e.g.,the empty volume 662 shown in FIG. 17 ) may improve the direct aircooling of the active parts, since the effective cooling surface of thecoils (e.g., the percentage of the coil surface area directly exposed tothe air) may be increased. However, this is balanced against thereduction in the slot current density that results from reducing theamount of conductor that is packed into the slot. Reducing the slot fillfactor (e.g., packing factor) too far may necessitate an increase in thecurrent, which may result in additional heat production (|²R) that isgreater than the additional heat removal capability of the coolingsystem gained from the reduced slot packing.

In accordance with the present disclosure, the coil design may beselected to tune the effective cooling surface area to optimize thebalance between direct air cooling efficiency and slot current density.For example, the number of winding packages, the number of turns perwinding package, the arrangement of turns within a winding package, theturn cross-section, the turn radius, or any combination thereof may beadjusted to optimize the effective cooling surface area. The effectivecooling surface area percentage, expressed as a percentage, is definedin Equation (29):

$\begin{matrix}{{{Effective}{cooling}{surface}{area}{}\%} = {\frac{{Exposed}{coil}{surface}{area}}{{Total}{coil}{surface}{area}} \times 100\%}} & (29)\end{matrix}$

In this equation, the total coil surface area is the sum of the surfaceareas of each winding turn of the coil. The exposed coil surface area isthe sum of the areas that are exposed to the cooling flow of air fordirect cooling.

FIGS. 23A and 23B illustrate how, for a given number of winding packagesand turns (e.g., in this case, two winding packages each having seventurns arranged in a trapezium shape), the effective cooling surface areamay be tuned by adjusting the conductor cross-section and radius.

In the first example, depicted in FIG. 23A, each turn 6140 of thewinding package 614 a-i has a circular cross-section. In the secondexample, depicted in FIG. 23B, each turn 6140 of the winding package 614a-i may have a rectangular cross-section with rounded corners. In bothcases, the turns 6140 are arranged in two rows: a first row of threeturns and a second row of four turns, wherein the turns are packed asclosely as possible. The effective cooling surface area of the windingpackage 614 a-i is the outer surface area of the winding package 614a-1, which is exposed to the flow of air. In both cases, the curvatureof the turn cross-section results in empty central regions 6145 that arenot occupied by conductor but are also inaccessible to cooling air, andthus do not form part of the effective cooling surface area.

Comparing the first and second examples, the empty central regions 6145are larger in the example in FIG. 23A than in the example in FIG. 23Bdue to the greater curvature of the turns in the example in FIG. 23A.This reduces the slot current density, which reduces torque production,without offering any improvement in cooling. However, portions of theeffective cooling surface areas adjacent regions A1, A2 (e.g.,corresponding to the air flow passages A1, A2 in FIG. 19A) are larger inthe example in FIG. 23A than in the example in FIG. 23B. Specifically,the rectangular shape of the turns in the example in FIG. 23B results inlinear, planar effective cooling surfaces adjacent regions A1, A2. Theresult of this is that the surfaces adjacent to the air flow passagesA1, A2 only expose one side of the rectangle (e.g., slightly over 25%)to the cooling air. In contrast, the curvature of the circular turns6140 in the example in FIG. 23A results in cooling surface areasadjacent regions A1, A2 that are non-linear. This exposes more of theturn surface area (e.g., slightly under 50%) to the cooling air.Overall, the example in FIG. 23B has a slightly higher packing factorand slot current density, resulting in slightly greater torqueproduction. However, the example in FIG. 23A has superior cooling due tothe greater cooling surface area adjacent the air passages A1, A2.

In accordance with the present disclosure, the effective cooling surfacearea may be at least 25% of the overall surface area of the coil. Valuesof between 35% and 70% may strike a particularly good balance betweencooling and torque production in a transverse flux motor.

For completeness, Table 13 summarizes the configuration and propertiesof a transverse flux electrical machine that is optimized for the use inthe EPU of a VTOL aircraft. This is merely one example and does notlimit the present disclosure to an electrical machine of thisconfiguration.

TABLE 13 Air-cooled 150 KW Dual-Lane Transverse Flux Motor Air gapconfiguration Double rotor, axial air gap Lane Configuration Two lanes,axially stacked active parts Cooling system type Direct air coolingContinuous rated power (P_(cont)) 150 kW Peak rated power (P_(peak)) 175kW Maximum continuous rated torque 1300 Nm Peak rated torque 1440 NmHover torque 975 Nm Rotor speed at maximum continuation 120 rads⁻¹ ratedtorque (ω_(max,cont)) Rotor speed at hover (ω_(hover)) 96 rads⁻¹ EPU tipspeed (v_(tip)) 171 ms⁻¹ Efficiency (η) 0.94 (94%) Maximum continuousrated current 200 A (RMS) Peak rated current 230 A (RMS) Steady-stateterminal short circuit 174 A (RMS) current Slot current density (peak)8.1 A/(mm)² Continuous rated voltage 900 V Active parts diameter 0.46meters Air gap 0.7 mm Active parts mass 15.2 kg Cooling system mass 4.2kg Total motor mass 56 kg Conductor volume 38.8 cm³ Iron volume 61.1 cm³${Pole}{pair}{number}( \frac{N_{P}}{2} )$ 80  ${Pole}{pitch}( {P_{\theta} = \frac{2\pi}{N_{P}}} )$ 0.039radians (2 degrees)${Pole}{arc}{length}( {P_{L} = \frac{P_{\theta} \times D_{Act}}{2}} )$8.97 mm Machine inductance 43 μH Power Factor (cos (Ø)) 0.72 Max.electrical frequency 1.4 kHz (f_(max)) Insulation rated temperature 475K (θ_(ins, max)) Coolant specific heat capacity 1006 Jkg⁻¹K⁻¹ (C_(p))Required mass flow rate at 0.27 kg τ_(max,cont) ({dot over(m)}_(max,cont)) C_(max,cont) = C_(p) × {dot over (m)}_(max,cont) 272JK⁻¹ $\rho_{act} = \frac{\tau_{peak}}{m_{act}}$ 94.7 Nmkg⁻¹$\rho_{{act} + {cool}} = \frac{\tau_{peak}}{m_{act} + m_{cool}}$ 74.2Nmkg⁻¹ $\Lambda = \frac{\tau_{peak}}{m_{act} \times J_{{slot},{peak}}}$11.7 μNm³kg⁻¹A⁻¹$\Lambda^{*} = \frac{\tau_{peak}}{( {m_{act} + m_{cool}} ) \times J_{{slot},{peak}}}$9.3 μNm³kg⁻¹A⁻¹ $\Gamma = \frac{V_{conductor}}{V_{iron}}$ 0.64$\Delta = \frac{\rho_{act}}{\cos(\varnothing)}$ 132 Nmkg⁻¹$\Delta^{*} = \frac{\tau_{peak}}{( {m_{act} + m_{cool}} ) \times {\cos(\varnothing)}}$103 Nmkg⁻¹ Υ = P_(θ) × G_(Air) 27.3 micro radian-meters Υ* = P_(L) ×G_(Air) 6.3 μm² $\Pi = \frac{P_{L}}{f_{\max}}$ 6.4 μms$\nabla = \frac{\tau_{\max,{cont}}}{m_{act} \times C_{\max,{cont}}}$0.31 Kskg⁻¹$\nabla^{*} = \frac{\tau_{\max,{cont}}}{( {m_{act} + m_{cool}} ) \times C_{\max,{cont}}}$0.24 Kskg⁻¹ $Z = \frac{{\cos(\varnothing)} \times m_{act}}{\eta}$ 11.6kg$Z^{*} = \frac{{\cos(\varnothing)} \times ( {m_{act} + m_{cool}} )}{\eta}$14.9 kg $\xi = \frac{I_{SC}}{I_{peak}}$ 0.76$\zeta = \frac{\theta_{{ins},{cont}}( I_{SC} )}{\theta_{{ins},{cont}}( I_{cont} )}$0.94 β = L_(machine) × ρ_(act) 4.1 mHNkg⁻¹$\lambda = \frac{\eta \times L_{machine}}{m_{act}}$ 2.7 μHkg⁻¹$\lambda^{*} = \frac{\eta \times L_{machine}}{( {m_{act} + m_{cool}} )}$2.1 μHkg⁻¹ F (see Equation (22)) 6.2 F* (see Equation (23)) 4.9$\chi = \frac{v_{tip} \times m_{act}}{\tau_{peak}}$ 1.8 sm⁻¹$\chi^{*} = \frac{v_{tip} \times ( {m_{act} + m_{cool}} )}{\tau_{peak}}$2.3 sm⁻¹ $\Psi = \frac{\tau_{hover}}{\omega_{hover}}$ 10.2 Nmsrad⁻¹Effective cooling surface area 42% Slot fill factor 28%

Various examples have been described, each of which features variouscombinations of features. It will be appreciated by those skilled in theart that, except where clearly mutually exclusive, any of the featuresmay be employed separately or in combination with any other features,and the disclosure extends to and includes all combinations andsub-combinations of one or more features described herein.

It is to be understood that the elements and features recited in theappended claims may be combined in different ways to produce new claimsthat likewise fall within the scope of the present disclosure. Thus,whereas the dependent claims appended below depend on only a singleindependent or dependent claim, it is to be understood that thesedependent claims may, alternatively, be made to depend in thealternative from any preceding or following claim, whether independentor dependent, and that such new combinations are to be understood asforming a part of the present specification.

The invention claimed is:
 1. An electrical propulsion unit (EPU) for avertical take-off and landing (VTOL) aircraft, the EPU comprising: apropeller or a fan; and an electric motor comprising: a statorcomprising coils configured to carry current; and a rotor arranged tointeract with the stator, wherein the rotor is configured to generate atorque for driving rotation of the propeller or the fan, wherein thecoils of the stator have a cumulated conductor volume V_(conductor),wherein the stator, the rotor, or the stator and the rotor comprise aflux guiding iron material configured to guide magnetic flux generatedby the coils of the stator, wherein the flux guiding iron material has acumulated iron volume V_(iron), and wherein the electric motor has amachine parameter Γ that is greater than or equal to 0.25, F beingdefined as: $\Gamma = {\frac{V_{conductor}}{V_{iron}}.}$
 2. The EPU ofclaim 1, wherein the machine parameter Γ is less than or equal to 3.0.3. The EPU of claim 1, wherein the machine parameter Γ is in a range of0.3 to 1.0.
 4. The EPU of claim 1, wherein the machine parameter Γ is ina range of 0.35 to 0.9.
 5. The EPU of claim 1, wherein the electricmotor is a transverse flux electric motor.
 6. The EPU of claim 5,wherein the rotor of the transverse flux electric motor is ironless. 7.The EPU of claim 5, wherein the rotor of the transverse flux electricmotor is a dual rotor comprising a first rotor portion and a secondrotor portion spaced apart from the first rotor portion, the first rotorportion comprising a first set of permanent magnets and the second rotorportion comprising a second set of permanent magnets, and wherein thestator is located between the first rotor portion and the second rotorportion.
 8. The EPU of claim 7, wherein the first rotor portion and thesecond rotor portion are axially spaced apart, and the stator isprovided axially between the first rotor portion and the second rotorportion.
 9. The EPU of claim 5, wherein the stator of the transverseflux electric motor comprises the flux guiding iron material definingone or more stator slots housing the coils, wherein each respectivestator slot of the one or more stator slots is a circumferentiallyextending slot, a respective coil of the coils of the stator beingconfigured, such that current flows through the respective coil in acircumferential direction, and wherein the rotor of the transverse fluxelectric motor comprises a plurality of permanent magnetscircumferentially distributed about the rotor.
 10. The EPU of claim 9,wherein, for each respective stator slot of the one or more statorslots, the flux guiding iron material comprises a plurality ofcircumferentially arranged stator elements.
 11. The EPU of claim 10,wherein each stator element of the plurality of circumferentiallyarranged stator elements comprises an axially extending body portion anda pair of axially spaced and radially extending projections that projectfrom the axially extending body portion.
 12. The EPU of claim 5, whereinthe transverse flux electric motor is a multi-lane transverse electricmotor comprising a first sub-machine and a second sub-machine, whereinthe first sub-machine comprises a first stator having first coilsconfigured to carry current and a first rotor arranged to interact withthe first stator, such that a first torque for driving rotation of thepropeller or the fan is produced, wherein the second sub-machinecomprises a second stator having second coils configured to carrycurrent and a second rotor arranged to interact with the second stator,such that a second torque for driving rotation of the propeller or thefan is produced, wherein the first stator and the first rotor arecoaxial with and axially spaced apart from the second stator and thesecond rotor, wherein the cumulated conductor volume V_(conductor) is acumulated volume of the first coils of the first stator and the secondcoils of the second stator, and wherein the cumulated iron volumeV_(iron) is a cumulated volume of the flux guiding iron material of thefirst stator, the first rotor, the second stator, and the second rotor.13. The EPU of claim 1, wherein the cumulated conductor volumeV_(conductor) is in a range of 15 to 80 cm³.
 14. The EPU of claim 1,wherein the cumulated iron volume V_(iron) is in a range of 35 to 120cm³.
 15. The EPU of claim 1, wherein a peak rated power output of theelectrical motor is in a range of 60 kW to 450 kW.
 16. A verticaltake-off and landing (VTOL) aircraft comprising: an electricalpropulsion unit (EPU) comprising: a propeller or a fan; and an electricmotor comprising: a stator comprising insulated conductive coilsconfigured to carry current; and a rotor arranged to interact with thestator and configured to generate a torque for driving rotation of thepropeller or the fan, wherein the insulated conductive coils have acumulated conductor volume V_(conductor), wherein the stator, the rotor,or the stator and the rotor comprise an iron material configured toguide a magnetic flux generated by the insulated conductive coils of thestator, wherein the iron material has a cumulated iron volume V_(iron),and wherein the electric motor has a machine parameter Γ that is greaterthan or equal to 0.25, F being defined as${\Gamma = \frac{V_{conductor}}{V_{iron}}}.$
 17. A transverse fluxelectric motor for an electrical propulsion unit (EPU) of an aircraft,the transverse flux electric motor comprising: a stator comprising:coils; and a flux guiding iron material, wherein the coils areconfigured to carry current, wherein the coils have a cumulatedconductor volume V_(conductor), wherein the flux guiding iron materialis configured to guide a magnetic flux generated by the coils, andwherein the flux guiding iron material has a cumulated iron volumeV_(iron); and an ironless rotor comprising a plurality of permanentmagnets circumferentially distributed about the ironless rotor andarranged to interact with the stator, such that a torque is generated,wherein the transverse flux electric motor has a machine parameter Γthat is greater than or equal to 0.25, Γ being defined as:${\Gamma = \frac{V_{conductor}}{V_{iron}}}.$